At a fairly early stage in the development of the Bell P-39 Airacobra, work had been carried out to enhance the performance of this aircraft by the introduction of aerodynamic improvements. Three experimental aircraft were built, each utilising the basic fuselage of the P-39D, to which were added a new laminar-flow wing with square wingtips and a revised tail unit. In fact, each of the three XP-39Es, as these aircraft were designated, had a different tail unit. It was planned originally to power the prototypes with the Continental Aviation and Engineering Corporation's IV-1430 12-cylinder inverted-Vee piston engine, which had demonstrated a power output in excess of 2,000 hp (1491 kW). However, Allison V-1710 engines of little more than half of that power output were installed, presumably because of unreliability of the Continental engine. Testing of the XP-39Es began in February 1942 and, proving satisfactory, the type was ordered into production under the designation P-76. Some 4,000 aircraft were to be built at Bell's Marietta, Ohio, facility but were cancelled only three months later.

It was decided, instead, to build a larger and more powerful version for utilisation in a close-support fighter and fighter-bomber role, and the research and design development which had been carried out for the XP-39E were used in finalising the design of what was to become known as the Ball Model 33, or P-63 Kingcobra. In its layout this latter aircraft was generally similar to the P-39, but apart from being larger and with the V-1710 engine more powerful than those installed in all but the P-39K and P-39L production aircraft, efforts had to be made to render this new development more suitable for the close-support role regarded as its primary mission.

Two prototypes were ordered by the US Army Air Corps in June 1941 under the designation XP-63, and these made their first flights on 7 December 1942 and 5 February 1943, both powered by the 1,325 hp (988 kW) Allison V-1710-47 engine. Both aircraft were lost in an early stage of their test programme, resulting in the construction of a third prototype, the XP-63A, first flown on 26 April 1943 and powered by a V-1710-93 engine with a war emergency rating of 1,500 hp (1119 kW). It was planned subsequently to flight-test this prototype with a Packard-Merlin V-1650-5 engine installed, under the designation XP-63B, but this did not happen.

The performance of the XP-63A was found to be satisfactory, and the type was ordered into production in September 1942. Initial deliveries of the P-63A began in October 1943, and by the time production ended in 1945 more than 3,300 Kingcobras had been built in several versions. By far the majority, something in excess of 2,400, were supplied to the USSR under lend-lease, and about 300 went to the Free French Armee de L'Air. Very few of the total production of P-63 close-support fighters/fighter-bombers were delivered to the USAAF, and so far as is known no Kingcobras were used operationally by that service.

Equipment of production batches varied considerably, resulting in many sub-types. The first production P-63A-1s had V-1710-93 engines, a nose-mounted 37 mm M4 cannon and two 12.7 mm (0.50 in) machine-guns in underwing fairings; other sub-types had two additional 12.7 mm (0.50 in) guns mounted in the fuselage nose. P-63A-1s and P63A-5s could accommodate a 284 litre (75 US gallon) or 662 litre (175 US gallon) drop tank, or a 522 lbs (237 kg) bomb beneath the wing centre-section; P-63A-6s had underwing racks for two similar bombs or additional fuel; and P-63A-10s could mount three air-to-surface rockets beneath each wing. The weight of defensive armour, intended primarily to give protection from ground weapons, increased progressively from 39.8 kg (87.7 lbs) on the P-63A-1 to 107.2 kg (236.3 lbs) on the P-63A-10.

The P-63A was succeeded on the production line by the P-63C with the V-1710-117 engine, this offering with water injection an emergency war rating of 1,800 hp (1342 kW). A distinctive identification feature of the P-63C was provided by the introduction of a small ventral fin. Other variants included a single P-63D with a V-1710-109 engine, a bubble canopy, and increased wing span; 13 of the P-63E (or Bell Model 41), all that had been produced of 2,930 on order when contracts were cancelled at the war's end, and which were generally similar to the P-63D except for a reversion to the standard cockpit canopy; and two P-63Fs, a version of the P-63E with a V-1710-135 engine and modified tail surfaces.

One other unusual version of the Kingcobra was built extensively (in excess of 300) for use by the USAAF in a training programme involving the use of live ammunition. Developed from the P-63A, all armour and armament was removed, and the external surface of the wings,  fuselage and tail unit were protected externally by the addition of a duralumin alloy skin weighing some 680kg (1,5001b). Other protection included the installation of bullet-proof glass in windscreen and cockpit side and upper windows, the provision of a steel grille over the engine air intake and steel guards for the exhaust stacks, and the use of a propeller with thick-walled hollow blades. All of these precautions were to make it possible for the aircraft to be flown as a target that could withstand, without significant damage, the impact of frangible bullets. When a hit was made by an attacking aircraft a red light blinked to confirm the accuracy of the weapon being fired against it.

The first five of these target aircraft were designated RP-63A-11; the 95 RP-63A-12s which followed had increased fuel tankage; the next production version, with the V-1710-117 engine, became designated RP-63C (200 built); and the final version was the RP-63G (32 built), this having the V-1710-135 engine. Although never flown as pilotless drone aircraft, the designations of these three versions were changed to QF-63A, QF-63C and QF-63G respectively.  

Specifications (Bell P-63A Kingcobra)

Type: Single Seat Fighter & Fighter Bomber

Design: Bell Aircraft Corporation

Manufacturer: Bell Aircraft Corporation

Powerplant: (P-63A) One 1,325 hp (988 kW) Allison V-1710-93 inline piston engine with up to 1,500 hp (1119 kW) using water-methanol boost (War Emergency Rating) at sea level. (P-63C) One 1,500 hp (1119 kW) V-1710-117 engine with up to 1,800 hp (1342 kW) using water-methanol boost (War Emergency Rating). (P-63D) One 1,425 hp (1063 kW) V-1710-109 engine. (P-63F) One 1,425 hp (1063 kW) V-1710-135 engine.

Performance: Maximum speed 410 mph (660 km/h) at 25,000 ft (7620 m); cruising speed 378 mph (608 km/h); service ceiling 43,000 ft (13110 m).

Range: With maximum weapon load and internal fuel 450 miles (724 km); ferry range with maximum internal and external fuel 2,200 miles (3541 km).

Weight: Empty 6,375 lbs (2892 kg) with a maximum take-off weight of 10,500 lbs (4763 kg).

Dimensions: Span 38 ft 4 in (11.68 m); length 32 ft 8 in (9.96 m); height 12 ft 7 in (3.84 m); wing area 248 sq ft (23.04 sq m).

Armament: One 37 mm M4 cannon with 30 rounds, two wing-mounted and two nose-mounted 12.7 mm (0.50 in) machine-guns, plus up to three 522 lbs (237 kg) bombs. (P-63A-9) Introduced the 37 mm M10 cannon with 58 rounds.

Variants: XP-63 (two prototypes), XP-63A (one prototype), P-63A, RP-63A-11/12 (later designated Q-63A), P-63B (never built), P-63C, RP-63C (later designated QF-63C), P-63D, P-63E, P-63F, RP-63A/C "Pinball", RP-63G (later designated QF-63G), L-39 (swept wing, TP-63 (two seat), P-63 (vee tail), XP-63H.

Avionics: None.

History: First flight (XP-63) 7 December 1942; service delivery October 1943; final delivery early 1945.