At night when day fighters were sleeping,
The nocturnal raids were begun.
And the heavens were sore split asunder,
By the roar of our P sixty-ones.
—from The Night Fighter’s Lament, author unknown. In Jeff Kolln, The 421st Night Fighter Squadron in World War II (Schiffer 2001).
Most pilots enjoy flying at night. After sunset, the air turns silky smooth as the bumps and jumps of turbulence caused by the Sun’s uneven heating of the Earth give way to calm. The airplane seems suspended on a string as the world moves around it. The smooth night air conferred no advantage to either attacker or defender during World War II, but the inability to spot darkened objects at a distance presented unique challenges. In daylight, keen-eyed pilots could see other aircraft many miles away and could protect each other by flying in close formations. At night, the risks of collision and friendly fire forced night fighter crews to fly as single aircraft without wingmen. To successfully engage enemy aircraft, they had to find them and positively identify them as enemy aircraft before attacking.
To detect approaching night attackers, military forces tried out searchlights—both ground-based and mounted on aircraft—acoustic listening devices, infrared detectors to spot the hot engine exhaust, and devices to detect the electric charge from engine spark plugs. Ground-based radar became the best tool, but it was not always available in remote combat areas. Assigning night fighters to patrol designated sectors hoping to intercept enemy aircraft was successful but inefficient. Planners had to guess from where, at what altitude, and at what time a target was likely to appear, and this tactic required close coordination with anti-aircraft gun crews to prevent firing at the friendly patrolling night fighters.
Once a potential enemy aircraft, called a bogey, was detected, the night fighter crew had to maneuver close enough to identify it beyond doubt. Day fighter crews could often spot bogeys many miles distant. For the night fighters, even on nights illuminated by a full moon, detection ranges were usually less than 3 miles. Moonless nights reduced the range to less than 1,000 yards, and haze and clouds made the bogey all but invisible. Enemy aircraft crews took steps to hide their aircraft. They flew without external lights and used dim red cockpit lighting to reduce the chance that glowing instruments would expose their location and to maintain their night vision. Engineers modified engines to hide the glow from hot engine exhaust. When radar units small and lightweight enough to be carried by aircraft were developed in 1940, the combination of ground-based and airborne radar gave night fighter crews the best tools to locate and attack enemy aircraft at night.
When the Luftwaffe (German Air Force) began mounting small-scale night raids on Britain in June 1940, Royal Air Force (RAF) crews flying twin-engine Bristol Blenheim night fighters tried to stop them. The Blenheim carried four .303 caliber machine guns mounted in the aircraft’s belly and aimed toward the front of the aircraft. The RAF switched to using Boulton Paul Defiant fighters in August. The Defiant was a unique design equipped with a powered turret, housing a gunner and mounting four guns, fitted behind the pilot’s seat. The Blenheim’s belly guns and the Defiant’s turret were mounted out of sight of the pilot, so he was not blinded when they fired.
These aircraft required several crew members to fly the airplane, operate the radar, and control the turret. Both aircraft lacked speed and the performance of the airborne radar they carried was not very good. Nonetheless, key design features of the Blenheim and Defiant—twin engines, power turret, belly-mounted guns, and multiple crew members—found their place in the design of the Northrop P-61 Black Widow.
During negotiations in the fall of 1940 with the British Purchasing Commission to build a dive bomber for the RAF, John “Jack” K. Northrop discussed the preliminary ideas about a night fighter. In mid-October, the Luftwaffe launched about 400-night bombing sorties and London suffered heavy attacks. The Luftwaffe switched to bombing mostly at night in November and many attacks were against cities. The need was urgent for a night fighter with sufficient endurance to loiter above a target and wait for the bombers to approach. U. S. Army Air Corps Lt. Gen. Delos C. Emmons was in London during several attacks. He conveyed the need for an American night fighter to his Chief of Experimental Aircraft Projects, Col. Lawrence C. Craigie. On October 21, 1940, Craigie briefed Northrop’s chief of research, Vladimir H. Pavlecka on the need for an aircraft capable of carrying devices “that would locate aircraft in the dark.” Airborne radar was still too secret to reveal its true nature.
Initial Design to Final Design and Prototype Construction
The terms of the design contract offered to Northrop by the Army Air Corps in January 1941 described the requirements for a night fighter large enough to carry the SCR-720 radar plus a crew of three. Jack Northrop’s discussions with the British Purchasing Commission, and Col. Craigie’s brief to Pavlecka, informed the design of the prototype XP-61. A twin boom fuselage supported a central pod carrying a three-person crew consisting of a pilot, a gunner to operate a power turret and a specialist to run the radar. Many wartime publications refer to the radar operator as a “radio observer” to protect the secrecy of the airborne radar technology.
The pod also held the armament, a powered, four-gun turret on top, and four cannons in the belly of the pod firing forward. Combat experience had shown that the pilot of an aircraft targeted by a night fighter could often elude the first attack by maneuvering violently. The night fighter’s heavy armament helped ensure the target did not escape. The pilot and gunner sat under a stepped canopy which allowed the gunner to see ahead and above the airplane to aim the turret mounted behind him. In the Boulton Paul Defiant, the gunner had occupied the turret.
Several design criteria favored a twin-boom design. An important consideration was providing each crew member an unobstructed view outside the airplane. Pilot and gunner could see forward well enough, and a clear bubble canopy fitted to the aft end of the nacelle, allowed the radar operator to scan for targets behind the XP-61. The Army’s specifications required that the turret fire control system allow any of the three crew members to control the turret if they spotted a target. British experts also thought the twin-boom layout enhanced stability and control as the night fighter closed in on an adversary to make a positive ID before attacking. The twin-boom configuration required a tricycle landing gear because it was impractical to employ a tailwheel on the aft end of each boom.
The XP-61 was as large as the B-25 Mitchell and B-26 Marauder medium bombers, and Northrop needed powerful engines to make the Black Widow agile enough to chase down enemy aircraft. Engineers selected two 18-cylinder Pratt & Whitney R-2800 air-cooled radials that could each make about 2,000 hp. These engines lacked turbosuperchargers and this caused performance to suffer at high altitudes.
Getting all the components of such a complex aircraft to fit and function satisfactorily required more than a year of careful design work, however on January 30, 1942, the Army Air Corps awarded Northrop a contract to build two XP-61 prototypes. By spring, it was time to fly. One area of concern was the unorthodox method devised for rolling the big fighter. Large flaps installed on the wing left no space to fit conventional ailerons, so the design team used spoilers. The flaps had to be very large to meet the army requirement for the P-61 to approach and land slowly at night and in bad weather. Concerns turned out to be unfounded – the spoilers worked well. Landing after the first flight on May 26, 1942, contract test pilot Vance Breeze declared that the XP-61 "flies beautifully and is an old man's airplane," meaning that it was so docile that even an old man could fly it. The success of the prototype seemed to assure a production contract would soon be forthcoming. Now, the aircraft needed a catchy name. Someone saw the prototype painted shiny gloss black with red serial numbers and inspection door markings – and named it Black Widow.
Part II of the blog describes the combat history of P-61 night fighters, and the flight history of the Museum’s P-61C Black Widow (coming soon).