The North American P-82 Twin Mustang was the last piston-engined fighter ordered by the USAAF. It was conceived late in 1943, when it was realized that the very long distances over which single-seat fighters were operating in the Pacific were imposing very great strains on their pilots. It was not uncommon for fighter pilots to spend up to eight hours in their cockpits on maximum-distance missions. Pilots would often return from such missions drenched in sweat and would be so exhausted that they would have to be lifted from their cockpits.
It seemed that a fighter with a second seat for a copilot might be a good idea. North American Aviation came up with the idea of joining two Mustang fuselages together by a constant-chord wing midsection and a rectangular tailplane, using standard port and starboard outer wings. The project was given the company designation of NA-120.
On January 7, 1944, the USAAF ordered four prototypes under the designation XP-82. The twin fuselages were basically similar to that of the P-51H, but were lengthened by some 57 inches by inserting additional sections with integral dorsal fins in front of the tailplane. All parts of the wing had to be completely redesigned internally to carry the very much greater gross weight and to accommodate the increased fuel capacity. The center wing section carried a full set of flaps and was stressed to carry heavy external loads on either one or two pylons. The outer wings were stressed for two pylons. Because of the greater rolling inertia, each aileron was increased in length and divided into inner and outer sections to prevent binding of the hinges under high g-loads.
The engines were 1860 hp Packard Merlin V-1650-23/25 liquid-cooled Vees driving oppositely-rotating Aeroproducts propellers. Each main undercarriage leg was attached to the front wing spar under the outboard side of each fuselage. The wheels retracted inwards under the fuselage and wing. Armament consisted of six 0.5-inch Browning MG 53-2 machine guns with 300 rpg mounted in the new wing center section. These guns could be augmented by a central pod below the wing, with 8 additional machine guns installed.
The two cockpits with dual controls had the same bubble canopies as the P-51H. The port cockpit housed the pilot and was equipped with a full set of flight and engine instruments. The starboard cockpit housed the navigator/copilot and was equipped with only a limited set of instruments, sufficient only for basic control of the plane. At an early stage, it was suspected that once the war was over, the Packard Motor Car Company would be unlikely to continue the manufacture of V-1650 Merlin engines. Furthermore, the Allison V-1710 had by that time matured into an engine which, without a turbosupercharger but with two stages of gear-driven blowers could give excellent performance at high altitudes. Consequently, the V-1710-119 was specified for the third and fourth prototypes, which were to be designated XP-82A. XP-82 serial number 44-83887 flew for the first time on April 15, 1945, piloted by J. E. Barton. The other XP-82 (44-83886) followed shortly thereafter. At first, there were problems with excessive drag, which was eventually traced to the fact that the propellers turned toward each other during their upward sweep. This tended to stall the center section of the wing. The problem was cured by switching the engines, so that the blades met during their downward trajectory. The performance was excellent, the XP-82 retaining all the excellent qualities of the P-51: high speed, excellent maneuverability, and heavy firepower. Maximum speed of the XP-82 was 468 mph at 22,800 feet. Normal range was 1390 miles, with maximum range being 2600 miles. Service ceiling was 40,000 feet, and an altitude of 25,000 feet could be attained in 6.4 minutes. Weights were 13,402 pounds empty, 19,100 pounds normal loaded, and 22,000 pounds maximum. Dimensions were wingspan 51 feet 3 inches, length 39 feet 1 inches, height 13 feet 10 inches, and wing area of 408 square feet.
Serial numbers of XP-82 Twin Mustang
44-83886/83887 North American XP-82 Twin Mustang c/n 120-43742/43743