The Northrop P-61 Black Widow was an all-metal, twin-engine, twin-boom, monoplane night fighter and night intruder aircraft flown by the United States Army Air Forces during World War II. It was the first American – and only Allied – purpose-built aircraft to serve as a radar-equipped night fighter.
During the few last years of peace leading up to World War II, both the Royal Air Force and the German Luftwaffe made the first and largest steps in developing and refining the theories of waging war with aircraft by night. The complexity and challenge of nocturnal airborne combat must be briefly explained to be fully appreciated, and to comprehend the undertaking and achievement the P-61 entailed.
First signs of the Black Widow
In August 1940, a full 16 months before the United States entered the war, the U.S. Air Officer in London, Lt. Gen. Delos C. Emmons, was briefed on British research in RADAR (Radio Detection and Ranging), which had been underway since 1936 and had played an important role in the nation's defense against the Luftwaffe during the Battle of Britain. Gen. Emmons was informed of the new Airborne Intercept radar (AI for short), a self-contained unit that could be installed in an aircraft and allow it to operate independently of ground stations. In September 1940 the Tizard mission traded British research on many aspects including radar for American production.
Simultaneously, the British Purchasing Commission evaluating US aircraft declared their urgent need for a high-altitude, high-speed aircraft to intercept the Luftwaffe night bombers attacking London. The aircraft would need to patrol continuously over the city throughout the night, requiring at least an eight-hour loiter capability. The aircraft would obviously be equipped with one of the early, and heavy, AI radar units. Specified armament was to be carried in \"multiple-gun turrets.\" The RAF’s Boulton Paul Defiant had proved capable as a radar-equipped night fighter but was succeeded in 1942 by the night fighter variants of the Bristol Beaufighter and de Havilland Mosquito. While excellent performers, each needed strengthening. The British conveyed the requirments for a new fighter to all the aircraft designers and manufacturers they were working with. Jack Northrop was among them, and he realized that the speed, altitude, fuel load and multiple-turret requirements demanded a large aircraft with multiple engines.
Gen. Emmons returned to the US with details of the British night-fighter requirements, and in his report said that US aircraft design bureaus possibly could produce such an aircraft. The Emmons Board developed basic requirements and specifications, handing them over towards the end of 1940 to Air Technical Service Command, Wright Field. After considering the two biggest challenges - the high weight of the AI radar and the very long (by fighter standards) loiter time of eight hours minimum - the board, like Jack Northrop, realized the aircraft would need the considerable power and resulting size of twin engines, and recommended such parameters.
Vladimir H. Pavlecka, Northrop Chief of Research, was present on unrelated business at Wright Field. On October 21, 1940, Col. Lawrence Craigie of the ATSC phoned Pavlecka, explaining the USAAC's specifications, but told him to \"not take any notes, 'Just try and keep this in your memory!'\" (Davis & Menard, 4). What Pavlecka did not learn was radar's part in the aircraft; Craigie described the then super-secret radar as a \"device which would locate enemy aircraft in the dark\" and which had the capability to \"see and distinguish other airplanes.\" The mission, Craigie explained, was \"the interception and destruction of hostile aircraft in flight during periods of darkness or under conditions of poor visibility.\"
Pavlecka met with Jack Northrop the next day, and gave him the USAAC specification. Northrop compared his notes with those of Pavlecka, saw the similarity between the USAAC's requirements and those issued by the RAF, and pulled out the work he had been doing on the British aircraft's requirements. He was already a month along, and a week later, Northrop pounced on the USAAC proposal.
On November 5, Northrop and Pavlecka met at Wright Field with Air Material Command officers and presented them with Northrop’s preliminary design. Douglas’ XA-26A night fighter proposal was the only competition, but Northrop’s design was selected and the Black Widow was conceived.
Following the USAAC acceptance, Northrop began comprehensive design work on the aircraft to become the first to design a dedicated night fighter. The result was the largest and one of the most deadly pursuit-class aircraft flown by the US during the war.
Jack Northrop's first proposal was a long fuselage gondola between two engine nacelles and tail booms. Engines were Pratt & Whitney R2800-10 Double Wasp 18-cylinder radials, producing 2,000 horsepower (1.5 MW) each. The fuselage housed the three-man crew, the radar, and two four-gun turrets. The guns -- .50 cal (12.7 mm) Browning M2s -- were fitted with 36 inch \"aircraft\" barrels with perforated sleeves. The turrets were located in the nose and rear of the fuselage. It stood on tricycle landing gear and featured full-span retractable flaps, or \"Zap flaps\" (named after Northrop engineer Edward Zap) in the wings.
The aircraft was huge, as Northrop had suspected. While far heavier and larger multi-engine bombers existed, the 45 foot, 6 inch length, 66 foot wingspan and projected 22,600 lb full-load weight were unheard of for a fighter making the P-61 hard for many to accept as a feasible combat aircraft.
Changes to the plan
Some alternative design features were investigated before finalization. Among them was conversion to a single vertical stabilizer/rudder but this was dismissed. One change kept was the shifting of the nose and tail gun turrets to the top and bottom of the fuselage and the incorporation of a second gunner.
Late in November 1940, Jack Northrop returned to the crew of three and twin tail/rudder assembly. To meet USAAC's request for more firepower, the belly turret was abandoned and four 20 mm Hispano M2 cannons were mounted in the wings. The P-61 therefore became one of the few US designed aircraft to have 20 mm cannons as factory-standard. Others were the P-38, the F4U-1C, a limited production Corsair sub-variant, and the A-36 Apache dive-bomber, an early form of the P-51 Mustang. While some F6F Hellcats and repossessed British lend-lease P-39s (renamed as P-400s) were also fitted with 20 mm cannons, it was not standard practice.
Northrop Specification 8A was formally submitted to Army Air Material Command at Wright Field, on December 5, 1940. Following a few small changes, Northrop's NS-8A fulfilled all USAAC requirements. Northrop was issued a Letter of Authority For Purchase on December 17. A contract for two prototypes and two scale models to be used for wind tunnel testing, (costs not to exceed $1,367,000), was awarded on January 10, 1941. Northrop Specification 8A became, by designation of the Department of Defense, the XP-61.
The P-61 featured a crew of three: pilot, gunner and radar operator. It was armed with four 20 mm Hispano M2 forward firing cannons, mounted in the lower fuselage, and four Browning M2 .50 cal (12.7 mm) machine guns, lined up horizontally with the two middle guns slightly offset upwards in a remotely-aimed turret, dorsally mounted. The turret was driven by the General Electric GE2CFR12A3 gyroscopic fire control computer, and could be directed by either the gunner or radar operator, who both had the aiming control and gyroscopic collimator sight assembly posts attached to their swiveling seats.
The two Pratt & Whitney R2800-25S Double Wasp radial engines were each mounted approximately one-sixth out on the wing's span. Two-stage, two-speed mechanical superchargers were fitted. No turbo-superchargers were fitted, despite the expected 50 mph top speed and 10,000 ft operational ceiling increases, as they were not felt to be necessary, would have occupied considerable space and added significant weight.
Main landing gear bays were located at the bottom of each nacelle, directly behind the engine. The two main gear legs were each offset significantly towards outboard in their nacelles, and retracted towards the tail; oleo scissors faced forwards. Each main wheel was inboard of its gear leg and oleo. Main gear doors were two pieces, split evenly, longitudinally, hinged at inner door's inboard edge and the outer door's outboard edge.
Each engine cowling and nacelle drew back into tail booms that terminated upwards in large vertical stabilizers and their component rudders, each of a shape similar to a rounded right triangle. The leading edge of each vertical stabilizer was faired smoothly from the surface of the tail boom upwards, swept back to 37 degrees. The horizontal stabilizer extended between the inner surfaces of the two vertical stabilizers, and was approximately three-quarters the chord of the wing root, including the elevator. The elevator spanned approximately one third of the horizontal stabilizer's width, and in overhead plan view, angled inwards in the horizontal from both corners of leading edge towards the trailing edge approximately 15 degrees, forming the elevator into a wide, short trapezoid. The horizontal stabilizer and elevator assembly possessed a subtle airfoil cross-section.
The engines and nacelles were outboard of the wing root and a short 'shoulder' section of the wing that possessed a four degrees of dihedral, and were followed by the remainder of the wing which had a dihedral of two degrees. The leading edge of the wing was straight and perpendicular to the aircraft's centerline. The trailing edge was straight and parallel to the leading edge in the shoulder, and tapered forward 15 degrees outboard of the nacelle. Leading edge updraft carburetor intakes were present on the wing shoulder and the root of the outer wing, with a few inches of separation from the engine nacelle itself. They were very similar in appearance to those on the F4U Corsair– thin horizontal rectangles with the ends rounded out to nearly a half-circle, with multiple vertical vanes inside to direct the airstream properly.
No ailerons were present. Aside from the full-span retractable \"Zap flaps,\" all control of the aircraft about the roll axis was maintained through the use of curved, tapered spoilerons, of approximately ten feet in length and six inches in width (in overhead plan view) each. They were located outboard of the outer edge of each nacelle in overhead plan view, approximately one-quarter the length of the outer wing (the section of wing outboard of the edge of each nacelle furthest from the aircraft's centerline) and offset towards the wing leading edge approximately one third the wing's chord from the trailing edge, running towards the wing-tip approximately half the length of the outer wing. Operation was as follows: the spoileron in the wing being turned towards rotated out of the wing's upper surface into the airstream, disrupting effects due to Bernoulli's principle and reducing lift over that wing, causing it to drop.
The main fuselage, or gondola, was centered on the aircraft's centerline. It was, from the tip of the nose to the end of the Plexiglas tail-cone, approximately five-sixths the length of one wing (wing root to wing tip). The nose housed an evolved form of the SCR-268 Signal Corps Radar, the Western Electric Company's SCR-720A. Immediately behind the radar was the forward crew compartment, seating the pilot and behind him the gunner, the latter elevated approximately six inches. The multi-framed \"greenhouse\" canopy featured two distinct levels, one for the pilot and a second for the gunner above and behind him. Combined with the nearly flat upper surface of the aircraft's nose, the two-tiered canopy gave the aircraft's nose a distinct appearance of three wide, shallow steps. The forward canopy in the XP-61 featured contiguous, smooth-curved, blown-Plexiglas canopy sections facing forward, in front of the pilot and the gunner. The tops and sides were framed.
Beneath the forward crew compartment was the nose gear wheel well, which the pilot and gunner entered and exited the aircraft through. The forward gear leg retracted to the rear, up against a contoured cover that when closed for flight formed part of the cockpit floor; the gear would not have space to retract with it open. The oleo scissor faced forwards. The nosewheel was centered, with the strut forking to the aircraft's left. The nosewheel was approximately three-fourths the diameter of the main wheels. Nose gear doors were two pieces, split evenly, longitudinally, hinged at each outboard edge.
The center of the gondola housed the main wing spar, fuel storage, fuel piping and control mechanisms, control surface cable sections, propellor and engine controls, and radio/IFF (Identification Friend or Foe) /communications equipment, but was predominantly occupied by the top turret mounting ring, rotation and elevation mechanisms, ammunition storage for the turret's four Browning M2 .50 cal (12.7 mm) heavy machine guns, the GE2CFR12A3 gyroscopic fire control computer, and linkages to the gunner and radar operator's turret control columns, forward and aft, respectively.
At the aft of the gondola was the radar operator's station. Entered by way of a small hatch with a built-in ladder on the underside of the aircraft, the SRC-720 radar set was controlled and its display scopes viewed from the isolated rear compartment. Intercom and radio controls, as well as the radar operator's controls and sight for the remote turret, were present in addition to the radar systems themselves. The compartment's canopy followed the curvature of the gondola's rear section, with only a single rounded step to the forwards canopy's double step. The rear of the gondola was enclosed by a blown Plexiglas cap that tapered quickly in overhead plan view to a barely-rounded point; the shape was somewhat taller in side profile than it was in overhead plan view, giving the end of the \"cone,\" a rounded \"blade\" appearance when viewed in perspective.
The cross-section of the gondola, front to back, was generally rectangular, vertically oriented. The tip of the nose was very rounded, merging quickly to a rectangular cross-section that tapered slightly towards the bottom. This cross-section lost its taper but became clearly rounded at the bottom moving back through the forward crew compartment and nose gear well. Height increased at both steps in the forward canopy, with the second step being flush with the top of the aircraft (not counting the spinal gun turret). At the rear of the forward crew compartment, the cross-section's bottom bulged downwards considerably and continued to do so until just past the midpoint between the rear of the forward crew compartment and the front of the rear crew compartment, where the lower curvature began to recede. Beginning at the front of the rear crew compartment, the top of the cross-section began to increasingly taper inwards above the aircraft's center of gravity when progressing towards the rear of the gondola. The cross-section rounded out considerably by the downward step in the rear canopy, and rapidly became a straight-sided oval, shrinking and terminating in the tip of the blown-Plexiglas \"cone\" described above.
The cross-section of the nacelles was essentially circular throughout, growing and shrinking in size when moving from the engine cowlings past the wing and gear bay, towards the tail booms and the vertical stabilizers. A bulge on the top of the wing maintained the circular cross-section as the nacelles intersected the wing. The cross-section became slightly egg-shaped around the main gear bays, larger at the bottom but still round. An oblong bulge on the bottom of the main gear doors, oriented longitudinally, accommodated the main wheels when the gear was retracted.
Wing tips, wing-to-nacelle joints, tips and edge of stabilizers and control surfaces (excluding the horizontal stabilizer and elevator) were all smoothly rounded, blended or filleted. The overall design was exceptionally clean and fluid as the aircraft possessed very few sharp corners or edges.
In March 1941, the Army/Navy Standardization Committee decided to standardize use of updraft carburetors across all U.S. military branches. The XP-61, designed with downdraft carburetors, faced an estimated minimum two month redesign of the engine nacelle to bring the design into compliance. The updraft carburetor standardization decision was later reversed by the committee (the XP-61 program's predicament having little likely influence), preventing a potential setback in the XP-61's development.
The Air Corps Mockup Board met at Northrop on April 2, 1941, to inspect the XP-61 mockup. Several changes were made following the review. Most prominently, the four 20 millimeter Hispano M2 cannon were relocated from the outer wings to the belly of the aircraft, clustered tightly just behind the rear of the nose gear well. The closely spaced, centered installation, with two cannons stacked vertically, slightly outboard of the aircraft's centerline on each side, and the top cannon in each pair only a few inches father outboard, eliminated the inherent drawbacks of convergence.
Convergence was a necessity in wing-mounted guns--the specific point or points of range and elevation at which the weapons' projectile paths were calibrated to intersect the aircraft's centerline, preventing the \"safe zone\" in front of the aircraft which no projectiles would pass through if wing guns were set to fire straight ahead. Weapons fire at a target beyond the point of convergence crisscrossed behind the target and missed wide; fire at a target closer than the point of convergence either passed on either side or failed to impact in a concentrated point, preventing maximum damage from being inflicted. Both cases limited the cannons' effective ranges to a very small zone on either side of a set distance, and created additional challenges when calculating deflection (\"pulling lead\") for a moving target.
Without convergence, aiming was considerably easier and faster, and the tightly grouped cannons created a thick stream of 20 mm projectiles. The removal of the guns and ammunition from the wings also cleaned up the wings' airfoil and increased internal fuel capacity from 540 to 646 gallons.
Other changes included the provision for external fuel carriage in drop tanks, flame arrestors/dampeners on engine exhausts, and redistribution of some radio equipment. While all beneficial, especially the movement of the cannons, the modifications required over a month of redesign work, and the XP-61 was already behind schedule.
In the summer of 1941, the dorsal turret mount finally proved too difficult to install in the aircraft, and was changed from the General Electric ring mount to a pedestal mount like that used for the upper turrets in B-17s, B-24s, B-25s, A-20s and other bombers. Following this modification, the turret itself became unavailable, as operational aircraft– in this case, the B-29– were ahead of experimental aircraft in line for the high-demand component. A dummy turret was used for flight testing.
During February, 1942, Northrop was notified by manufacturer Curtiss that the C5424-A10 four-bladed, automatic, full-feathering propellor Northrop had planned for use in the XP-61 would not be ready for the prototype rollout or the beginning of flight tests. Hamilton Standard propellors were used in lieu of the Curtiss props until the originally planned type became available.
The XP-61's weight rose during construction of the prototype, to 22,392 lb empty and 29,673 lb at takeoff. Engines were R-2900-25S Double Wasp radials, turning 12 foot, two inch Curtiss C5425-A10 four blade propellors, both rotating clockwise when viewed from the front. Radios included two command radios, SCR-522As, and three other radio sets, the SCR-695A, AN/APG-1, and AN/APG-2. Central fire control for the gun turret was similar to that used on the B-29, the General Electric GE2CFR12A3.
The production model of the SCR-720A mounted a scanning radio transmitter in the aircraft nose; in Airborne Intercept mode, it was capable of a range of nearly five miles. The unit could also function as an airborne beacon / homing service, navigational aid, or in concert with interrogator-responder IFF (Identification Friend or Foe) units. The XP-61's radar operator located targets on his scope and steered the unit to track them, vectoring and steering the pilot to the radar target via oral instruction and correction. Once within range, the pilot used a smaller scope integrated into the main instrument panel to track and close on the target.
The XP-61's spine-mounted dorsal remote turret could be aimed and fired by any person of the three-man crew, or could be locked forward to be fired by the pilot in addition to the 20 mm cannons. The radar operator could rotate the turret to face to the rear, in order to engage targets behind the aircraft. Capable of a full 360 degrees rotation and 90 degrees elevation, the turret could conceivably be used to engage any target in the entire hemisphere above and to the sides of the XP-61.
P-61 in combat
The 6th NFS based on Guadalcanal received their first P-61s in early June, 1944. The aircraft were quickly assembled and underwent flight testing as the pilots transitioned from the squadron's aging P-70s. The first operational P-61 mission occurred on June 25. On June 30, 1944, the P-61 scored its first kill when a Japanese G4M Betty bomber was shot down.
In England, the 422nd NFS finally received their first P-61s in late June, and began flying operational missions over England in mid-July. These aircraft arrived without the dorsal turrets so the squadrons' gunners were reassigned to another NFS that was to continue flying the P-70. The first P-61 engagement in the European Theatre occurred on July 15 when a P-61 piloted by Lt. Herman Ernst was directed to intercept a V-1 Buzz Bomb. Diving from above and behind to match the V-1's 350mph speed, the P-61's plastic rear cone imploded under the pressure and the attack was aborted. The tail cones would fail on several early P-61A models before this problem was corrected. On July 16, Lt. Ernst was again directed to attack a V-1 and, this time, was successful, giving the 422nd NFS and the European Theatre its first P-61 kill.
Throughout the summer of 1944, P-61s operating in the Pacific Theatre would see sporadic action against Japanese aircraft. Most missions ended with no enemy aircraft sighted, but when the enemy was detected they were often in groups, with the attack resulting in multiple kills for that pilot and radar operator, who would jointly receive credit for the kill. Since pilots and radar operators did not always fly as a team, the kills of the pilot and radar operator were often different. On some occasions a pilot with only one or two kills would fly with a radar operator who was already an \"ace.\"
In early August 1944, the 422nd NFS transferred to Maupertus, France, and began meeting piloted German aircraft for the first time. A Bf 110 was shot down, and shortly afterwards, the squadron's commanding officer Lt. Colonel O. B. Johnson, his P-61 already damaged by flak, shot down a Fw 190. The 425th NFS scored its first kill shortly afterwards
In October 1944, a P-61 of the 422nd NFS, now operating out of an abandoned Luftwaffe airfield in Florennes, Belgium, encountered a Me 163 attempting to land. The P-61 tried to intercept it but the rocket plane was traveling too fast. A week later, another P-61 spotted a Me 262, but was also unable to intercept the jet. On yet another occasion, a 422nd P-61 spotted a Me 410 Hornisse flying at tree top level but, as they dove on it, the \"Hornet\" sped away and the P-61 was unable to catch it. Contrary to popular stories, no P-61 ever engaged in combat with a German jet or any of the late war advanced Luftwaffe aircraft. Most Luftwaffe aircraft types encountered and destroyed were Ju 188s, Bf 110s, Fw 190s, Do 217s and He 111s, while P-61 losses were limited to numerous landing accidents, bad weather, friendly fire and flak. Apart from an attack on a Bf 110 that turned against them, there were no reports of a P-61 being damaged by a German aircraft, and apart from one accidentally shot down by a RAF Mosquito, none were confirmed to be destroyed in aerial combat.
The absence of turrets and gunners in most European theatre P-61s presented several unique challenges. The 422nd NFS kept its radar operator in the rear compartment. This meant the pilot had no visual contact with the R/O. As a result, several courageous pilots continued flying their critically damaged P-61s under the mistaken belief that their R/O was injured and unconscious, when in fact the R/O had already bailed out. The 425th NFS had a more novel solution. They moved the R/O to the former gunner's position behind the pilot. This gave the pilot an extra set of eyes up front, and moved the plane's center of gravity about 15 inches forward, changing the plane's flight characteristics from slightly nose up to slightly nose down. This improved the P-61's overall performance.
By December 1944, P-61s of the 422nd and 425th NFS were helping to repel the German offensive know as the \"Battle of the Bulge,\" with two flying cover over the town of Bastogne. Pilots of the 422nd and 425th NFS switched their tactics from night fighting to daylight ground attack, strafing German supply lines and railroads. The P-61's four 20mm cannons proved highly effective in destroying large numbers of German locomotives and trucks.
By spring 1945, German aircraft were rarely seen and most P-61 night kills were Ju 52s attempting to evacuate Nazi officers under the cover of darkness.
The 422nd NFS produced three ace pilots, while the 425th NFS claimed none. Lt. Cletus \"Tommy\" Ormsby of the 425th NFS was officially credited with three victories. Unfortunately Lt. Ormsby was killed by friendly fire moments after attacking a Ju 87 on the night of March 24, 1945. His radar operator escaped with serious injuries and was saved by German doctors.
In the Pacific Theater in 1945, P-61 squadrons struggled to find targets. One squadron succeeded in destroying a large number of Kawasaki Ki-48 Lily Japanese Army Air Force twin engined bombers, another shot down several G4M Bettys, while another pilot destroyed two Japanese Navy Nakajima J1N1 Irving twin engined fighters in one engagement, but most missions ended with no enemy planes sighted. Several Pacific Theater squadrons finished the war with no confirmed kills at all. The 550th could only claim a crippled B-29 Superfortress, shot down after the crew had bailed out, leaving the plane on autopilot.
It is widely believed that the last two enemy aircraft destroyed before the Japanese surrender were both downed by a P-61 of the 548th NFS. This aircraft, known as \"Lady in the Dark\" was piloted by Lt. Lee Kendall, gaining its victories over a Ki-43 on the night of August 14/15, 1945, and a Ki-44 on the next night. However, this is incorrect; these were the last aircraft detroyed by a USAAF fighter; the last Japanese aircraft destroyed in World War II were by a Convair B-32, \"Hobo Queen Two,\" which destroyed two A6M Zeros on August 18, 1945.
On January 30, 1945 a lone P-61 performed a vital mission that was instrumental in the successful effort of the U.S. Rangers to free over 500, Japanese held, allied POW's at the Cabanatuan prison camp in the Philippines. As the Rangers crept up on the camp a P-61 swooped low and performed aerobatic maneuvers for several minutes. The distraction of the guards allowed the Rangers to position themselves, undetected, within striking range of the camp. The riveting story of the rescue and the role of the P-61 is told in the book Ghost Soldiers (by Hampton Sides) and in The Great Raid, a movie based upon the book.
In the Mediterranean Theatre, most night fighter squadrons transitioned from their aging Bristol Beaufighters into P-61s too late to achieve any kills in the \"Black Widow.\"
Had the P-61 appeared in theater several months earlier, the situation would have likely been different. Though the plane proved itself very capable against the majority of German aircraft it encountered, it was clearly outclassed by the new aircraft arriving in the last months of WWII. It also lacked external fuel tanks that would have extended its range, and saved many doomed crews looking for a landing site in darkness and bad weather. External bomb loads would also have made the plane more adaptable to the ground attack role it soon took on in Europe. These problems were all addressed eventually, but too late to have the impact they might have had earlier in the war. The plane proved very capable against all Japanese aircraft it encountered, but saw too few of them to make a significant difference in the Pacific war effort.