The Northrop P-61 Black Widow was the largest and heaviest fighter aircraft to enter service with the USAAF during the Second World War. It was also the first American aircraft specifically designed from the outset for the night fighting role. It made its operational debut in the South Pacific in the summer of 1944 and was the standard USAAF night fighter at the end of the war. Unlike other USAAF fighters such as the P-47 Thunderbolt or P-51 Mustang, the Black Widow did not chalk up a particularly impressive number of kills, because by the time of its entry into service, the Allies had already established almost overwhelming air superiority over virtually all fronts, and enemy aircraft were rather few and far between, especially at night.
The saga of the Northrop P-61 Black Widow begins back in August 1940, at the height of the Blitz on London. During this time, the US air officer in London, Lt. Gen. Delos C. Emmons, underwent a briefing in which he was brought up to date on British progress on radar (Radio Detecting and Ranging). Radar had first been developed in Britain in 1936, and British scientists and engineers were at that time working on the early versions of AI (Airborne Interception) radar sets which could be carried aboard airplanes, enabling them to detect and intercept other airplanes in flight without having to rely on ground installations.
At the same time, the British Purchasing Commission that was shopping for aircraft in the USA announced that they urgently required a night fighter that would be capable of stopping the German bombers that were attacking London by night. Such a fighter would have to be able to stay on station above London all night, which meant at least an 8-hour loiter time. In addition, the night fighter needed to have sufficient combat altitude in order to take on the bombers when they showed up.
When General Emmons returned to the USA, he reported that the British had an urgent need for night fighter aircraft, and that American industry might be able to supply that need. A preliminary specification was drawn up by the Emmons Board and was passed on to Air Technical Service Command at Wright Field in late 1940. Because of the heavy weight of the early AI radar and because of the high loiter time required, a twin-engined aircraft was envisaged.
Northrop Chief of Research Vladimir H. Pavlecka happened to be at Wright Field at that time on an unrelated project, and was told of the Army's need for night-fighters. However, he was told nothing about radar, only that there was a way to "see and distinguish other airplanes". He returned to Northrop the next day. On October 22, Jack Northrop met with Pavlecka and was given the USAAC's specification. At this time, no other company was known to be working on night fighters, although at about this time Douglas was starting work on their XA-26A night fighter and the AAC were considering the A-20B as an interim night fighter.
Northrop's proposal was a twin-engined monoplane powered by a pair of Pratt & Whitney R-2800 Double Wasp air-cooled radial engines mounted in low-slung nacelles underneath the wings. The nacelles tapered back into twin tail booms which were connected to each other by a large horizontal stabilizer and elevator. The long fuselage housed a crew of three. The crew consisted of a pilot, a gunner for the nose turret, and a radar operator/rear turret gunner. Each turret housed four 0.50-inch machine guns. A tricycle landing gear was fitted. Estimated weights were 16,245 pounds empty, 22,654 pounds gross. Height was 13 feet 2 inches, length was 45 feet 6 inches, and wingspan was 66 feet. These dimensions and weights were more typical of a bomber than a fighter.
On November 14, Northrop presented this revised design to the USAAC. An additional gunner's station was fitted. Nose and tail turrets of the original version were replaced by twin 0.50-in machine guns in the belly, and four 0.50-in machine guns in a dorsal turret. The crew was now up to four--a pilot, a radar operator, and two gunners. The airborne intercept radio was moved to the nose.
The design was revised still further on November 22. The belly turret was deleted, and the crew was changed back to three--pilot, gunner, and radar operator. The pilot sat up front, and the gunner sat immediately behind and above the pilot. The gunner was to operate the turret via remote control, using a special sight attached to a swiveling chair. A "stepped-up" canopy was used to provide a clear field of view for the gunner. The rear fuselage with its clear tail cone provided the radar operator with an excellent rearward view which enabled him to act as a tail gunner if the plane happened to be attacked from astern. Optionally, the dorsal turret guns could be "locked" into the forward-pointing position, so that they could be fired by the pilot. The belly guns were deleted, and four 20-mm cannon were to be fitted in the wings. This design was formalized into Northrop Specification 8A (or NS-8A), dated December 5, 1940.
Incorporated into the night fighter design was the Zap wing and Zap flap, named after Edward Zap, a Northrop engineer. These were attempts to increase the maximum lift coefficient and to decrease the landing speed by the use of improved lateral control and lifting devices
NS-8A was submitted to Wright Field. The Army was generally pleased with the design, but they suggested some changes. A letter of quotation prepared by Northrop for two experimental prototypes was presented to Materiel Command on December 17, 1940. Northrop signed the formal contract on January 11, 1941. A contract was let on January 30, 1941 for two prototypes and two wind-tunnel models. On March 10, 1941, a contract was issued for 13 YP-61 service test aircraft, plus one engineless static test airframe.
The mockup was ready for inspection in April of 1941. At that time, it was decided to move the four 20-mm cannon from the outboard portion of the wings to the belly. This was done to improve the ease of maintenance and to make the airflow over the wing smoother. The internal fuel capacity was increased from 540 gallons in two tanks to 646 gallons in four self-sealed tanks built into the wings.
In the meantime, development of the A/I radar had proceeded at a rapid pace. Radar development in the United States had been placed under the control of the National Defense Research Committee. The NDRC's Microwave Committee in turn had established the Radiation Laboratory at the Massachusetts Institute of Technology. The Radiation Laboratory was to handle the development of the XP-61's airborne interception (AI) radar. The designation of the radar was AI-10. The AI-10 radar was given the military designation SCR-520, where SCR stood for "Signal Corp Radio" (some references have this as standing for "Searchlight Control Radar"). The Western Electric corporation was assigned the responsibility of refining the design and undertaking the mass production of the radar.
In October 1941, a pedestal-type mount for the turret guns was substituted for the General Electric ring-type mount.
A letter of intent was initiated on December 24, 1941, which called for 100 P-61 production aircraft and spares. Fifty more were ordered on January 17, 1942. The order was increased to 410 aircraft on February 12, 1942, fifty of which were to be diverted to the RAF under Lend-Lease. The RAF order was eventually cancelled.
The XP-61 flew at Northrop Field for the first time on May 26, 1942, piloted by veteran contract test pilot Vance Breese. It was powered by a pair of Pratt & Whitney R-2800-10 radials of 2000 hp each. In keeping with its nocturnal role, it was finished in black overall, befitting its popular name that was taken from the poisonous North American spider. Wingspan was 66 feet, length was 48 feet 10 inches, and height was 14 feet 2 inches. Weights were 19,245 pounds empty, 25,150 pounds gross, and 28,870 pounds maximum. The aircraft was equipped with only a mockup of the top turret, as General Electric had not yet been able to deliver the real thing because of the higher priority of other projects.
The XP-61 had a maximum speed of 370 mph at 29,900 feet, and an altitude of 20,000 feet could be attained in 9 minutes. Service ceiling was 33,100 feet, and maximum range was 1450 miles.
In mid-June 1942, a new horizontal tail was designed to complement the full-span flaps. Eventually, the Zap flaps were completely eliminated because of their high cost and complexity of manufacture, and spoilers were added to supplement the conventional ailerons. The spoilers were located in the rear one-third of the wing, and were one of the most successful innovations introduced during the entire Black Widow program. Operating in conjunction with the conventional ailerons, the spoilers provided the desired rolling moment at speeds even below the stalling speed. Although the spoilers were fully capable of providing all necessary lateral control on their own, the ailerons were nevertheless still left on the airplane if only to provide "warm fuzzies" to pilots who were used to conventional ailerons.
On May 25, 1942, an agreement was reached between Northrop and the USAAC to produce 1200 P-61s at a government facility in Denver, Colorado. By the end of July, that order had been cut down to 207 aircraft and it was decided that the Northrop facilities at Hawthorne were to be used after all.
The thirteen YP-61s were delivered during August and September of 1943. In order to reduce vibrations from firing the 0.50-inch turret machine guns, some YP-61s were fitted with only two turret guns. The assignments of the YP-61s were varied. Some stayed at Northrop for flight testing and factory training of maintenance personnel. Some went to Wright Field in Ohio for service testing. Others went to Florida where they underwent operational suitability testing.
The YP-61s initially did not have any airborne interception radar fitted, but the SCR-520, a preproduction version of the SCR-720 which was to go into the production P-61A, was installed.