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In 1940, before the United States entered World War II, USAAF officers were briefed on British radar development, to include the development of airborne radar systems from which to intercept incoming bomber aircraft at night or during inclement weather. This meeting led to the development of one of the best night fighters of World War II, the Northrop P-61 Black Widow.

The P-61 was conceived as an all metal twin boom design, similar to the Lockheed P-38 Lightning, with a crew of three; pilot, gunner and radar operator. It was a large aircraft, in comparison to other fighters of the day with its length and wingspan larger than a number of medium bombers. Armed with four forward firing 20 mm cannons in the belly, which could be serviced from the ground without platforms, and four .50 caliber machine guns mounted in an upper rotating dorsal turret gave the Black Widow shocking firepower. The P-61 was powered by twin Pratt & Whitney R-2800 engines in nacelles which tapered into twin tail booms connected by a central horizontal stabilizer. At the time, the British were in need of a high-altitude, high-speed aircraft to intercept Luftwaffe bombers attacking London at night. The proposed aircraft would need an eight-hour loiter capability to effectively patrol the metropolitan area. The British also specified the aircraft to be armed with multiple gun turret(s) along with one of the early airborne radar units, which were relatively heavy. Jack Northrop, as well as General Delos Emmons of the USAAF, believed after meeting with British officials that such an aircraft was possible, but the speed, fuel load, altitude and multiple turret requirements dictated a large aircraft with multiple engines.

General Emmons, upon returning to the U.S. with a report on British night-fighter requirements, stated that he believed the design departments of American aircraft firms could produce such a plane. Emmons established a board to develop basic requirements and specifications, which turned over their results in late 1940 to the Air Technical Service Command (ATSC) at Wright Field, Ohio. After contemplating the two biggest challenges in designing the aircraft, the weight of the radar and the fuel requirement for a eight-hour loiter, Jack Northrop, as well as the review board, realized the aircraft would need considerable power, resulting in a twin-engine concept. The USAAF had two primary choices in engines, the Double Wasp and the Duplex Cyclone. Both engines began their development in the late 1930's and had the potential to achieve 2,000 hp. and had passed their initial flight tests. Though a number of senior Northrop executives were unaware of the radar aspect of the plane, the USAAF specifications paralleled those of the RAF closely enough for Northrop to make a contract proposal to the Air Material Command based upon its preliminary design, which was accepted by the AMC on Nov. 5, 1940. The Douglas XA-26A was the only competition.

Though the 20mm cannons were originally mounted in the wings, an inspection in April 1941 proved they were easier to service mounted in the belly of the plane. Fuel capacity was increased from 540 gallons in two wing tanks to 646 gallons in four wing-mounted self-sealing fuel tanks. The inaugural flight took place on May 26, 1942 without incident. While the P-61 resembled a medium bomber more than a fighter due to its size and weight, it was a relatively agile aircraft, capable of a number of maneuvers, with a gentle stall attitude. The Black Widow could even fly on one engine while fully loaded and had an excellent turning radius, giving a good account of itself against a number of single-engine fighters. Though the A model was highly maneuverable, it was slow when compared with first line fighters of the day. This speed deficiency would be corrected in the later B and C models. The P-61 featured innovations such as spoilers for lateral control, as well small ailerons in the wingtips to provide a more precise control. The P-61 also had an automatic pilot as well as a remote control gun turret mounted above the central fuselage. Both its weapons and radar were carried on the central fuselage, as opposed to other night fighter aircraft, in which the radar antennas extended out from the nose (which tended to slow the aircraft down while in flight). While the Black Widow was perhaps one of the best handling planes of the war, it was plagued with another problem. The plexiglass tail dome at the rear of the fuselage would sometimes tend to break off during high speed dives. This problem was corrected by strengthening the tail cone by replacing the welded magnesium alloy booms with more conventional aluminum alloy booms. This eliminated the majority of the tail cone issues. Early P-61s had an olive drab scheme with some aircraft painted olive drab on the top side with a flat black paint underneath. With both schemes easily detectable by searchlights, the USAAF decided upon a gloss black color, which was nearly invisible when flying past a searchlight beam.

For all of its weaponry and performance, the most important component on the P-61 was its radar. The early night-fighters lacked radar due to the bulk and weight of the units. However, with the British invention of the cavity magnetron in 1940, microwave radar came into being, more lighter and compact than the early units-and able to fit into an aircraft. A modified version of the British radar, the SCR-520 (Searchlight Control Radar) was developed by Bell Telephone Laboratories in conjunction with the Massachusetts Institute of Technology. In 1943 Western Electric was designated as the prime contractor for the SCR-720 radar system. The 720 was the primary night-fighter radar used by the Allies in World War II, which was similar to the AI Mark X in use by the British. The radar operator initially located targets on a large scope, guiding the pilot to the target. Once within range, the pilot used a smaller scope on the main instrument panel to approach the target. This scope had a range of about five miles.

In October 1943, the first production P61s began rolling off the assembly line. By May 1944 Black Widow units had arrived in Europe, with the P-61s initial combat role intercepting V-1 Buzz Bombs over England. After the D Day invasion, the Black Widows were diverted from their original mission to provide ground support attacks upon German trains and supply vehicles in both the front and rear areas. P-61 crews participated in the defense of Bastogne during the Battle of The Bulge, both patrolling that city from the air, as well as attacking German vehicles and supply lines. As the war progressed, P-61 groups found scarce contact with enemy aircraft, as reflected in the combat reports. The first P-61s reached the Pacific in June 1944. Flying from Guadalcanal, the Black Widows drew the first blood on June 30, in which a Black Widow shot down a Mitsubishi G4M Betty. Though additional P-61s arrived in theater over the summer, contact with enemy aircraft was limited. However, the P-61 had a claim to fame related to two other events before the end of the war. In January 1945, a P-61 assisted ground troops in the raid on the Cabanatuan prisoner of war camp in the Philippines by distracting the Japanese guards as the assault force neared. Though Japanese targets were virtually nonexistent to the end of the war, a P-61 was credited with scoring the final kill of the war when it downed a Nakajima Ki-44 Tojo on August 14/15. Though the P-61 served a short time in both major theaters, it performed a number of missions outside the role of night-fighter, proving the versatility of Northrop's design.

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