[3] In August 1940, sixteen months before the United States entered the war, the U.S. Air Officer in London, Lieutenant General Delos C. Emmons, was briefed on British research in radar ("Radio Detection And Ranging" as it was then known), which had been underway since 1935, and had played an important role in the nation's defense against the Luftwaffe during the Battle of Britain.

General Emmons was informed of the new Airborne Intercept radar (AI for short), a self-contained unit that could be installed in aircraft, and operated independently of ground stations.

In September 1940, the Tizard Mission traded British research, including the cavity magnetron that would make self-contained interception radar installations practicable, for American production.

Among those contacted by the British was Jack Northrop, who realized that the speed, altitude, fuel load, and multiple-turret requirements demanded a large aircraft with multiple engines.

[4] General Emmons returned to the U.S. with details of the British night-fighter requirements, and stated in his report that the design departments of the American aviation industry's firms could possibly produce such an aircraft.

The Emmons Board developed basic requirements and specifications, and handed them over to the Air Technical Service Command (ATSC) at Wright Field, Ohio towards the end of 1940.

After considering the two biggest challenges—the heavy weight of the AI radar, and the very long (by fighter standards) loiter time of eight hours minimum—the board, including Jack Northrop, realized the aircraft would need the considerable power and resulting size of twin engines, and recommended such parameters.

“The mission, Craigie explained, was ‘the interception and destruction of hostile aircraft in flight during periods of darkness or under conditions of poor visibility.’”[citation needed] Pavlecka met with Jack Northrop the next day, and gave him the USAAC specifications.

Most prominently, the four 20 mm M2 cannons were relocated from the outer wings to the belly of the aircraft, clustered tightly with the forward-facing ventral "step" in the fuselage to accommodate them being placed just behind the rear edge of the nose gear well.

Other changes included: the provision for external fuel carriage in drop tanks; flame arrestors/dampers on engine exhausts; and redistribution of some radio equipment.

It was armed with four 20 mm (.79 in) Hispano M2 forward-firing cannon mounted in the lower fuselage, and four .50 in (13 mm) M2 Browning machine guns lined up horizontally with the two middle guns slightly offset upwards in a remotely aimed dorsally mounted turret, a similar arrangement to that used with the Boeing B-29 Superfortress using four-gun upper forward remote turrets.

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.

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.

The tip of the nose was very rounded to accommodate the main AI radar's dish antenna, merging quickly to a rectangular cross-section that tapered slightly towards the bottom.

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 then diminishing in size when moving from the engine cowlings past the wing and gear bay, towards the tail booms and the vertical stabilizers.

The production model of the SCR-720 mounted a scanning radio transmitter in the aircraft nose; in Airborne Intercept mode, it had a range of nearly 5 mi (8.0 km).

A brief assessment of the turret by the British Aeroplane & Armament Experimental Establishment in 1944 found problems with the aiming and "jerky movement" of the guns.

believed the P-61 was too slow to effectively engage German fighters and medium bombers, a view which the RAF shared, based on the performance of a single P-61 they had received in early May.

Pilots of the 422nd and 425th NFS switched their tactics from night fighting to daylight ground attack, strafing German supply lines and railroads.

Poet and novelist James Dickey flew 38 Pacific Theater missions as a P-61 radar operator with the 418th Night Fighter Squadron, an experience that influenced his work, and for which he was awarded five Bronze Stars.

Historian Warren Thompson wrote that "it is widely believed" that the last enemy aircraft destroyed in combat before the Japanese surrender was downed by a P-61B-2 named "Lady in the Dark" (s/n 42-39408) of the 548th NFS.

It also lacked external fuel tanks until the last months of the war,[20] an addition that would have extended its range and saved many doomed crews looking for a landing site in darkness and bad weather.

[2][page needed] The useful life of the Black Widow was extended for a few years into the immediate postwar period due to the USAAF's problems in developing a useful jet-powered night/all-weather fighter.

Under Third Air Force they were engaged in Weather Reconnaissance training immediately after the war, but the rapid demobilization of the AAF led to the 57th being inactivated by the end of the year, and 58th followed suit in May 1946.

Scientists from several universities also helped launch, design, and conduct of the project, which aimed to learn more about thunderstorms and how to better protect civil and military airplanes from them.

The P-61's radar and particular flight characteristics enabled it to find and penetrate the most turbulent regions of a storm, and return crew and instruments intact for detailed study.

Results derived from this pioneering field study formed the basis of the scientific understanding of thunderstorms, and much of what was learned has been changed little by subsequent observations and theories.

Replacement of the P-61 began in 1948 with F-82 night fighters, and by the end of the year all of the ADC Black Widows in the United States, Alaska and in Panama were off the inventory rolls.

[23] In 1948, the National Advisory Committee for Aeronautics (NACA) obtained a P-61C from Air Research and Development Command for a series of drop tests of swept-wing aerodynamic drones at Moffett Field, California.