The P-63D was the next progressive development of the Kingcobra series. It featured an Allison V-1710-109 (E22) engine rated at 1425 hp for take off. The wing (later adopted for the P-63E) had a ten-inch increase in span to 39 feet 2 inches, gross area being increased to 255 square feet. The P-63D also differed from the late production blocks of the P-63C in not having the rear ventral fin extension. However, the most noticeable new feature of the P-63D was the use of a rearward-sliding bubble canopy in place of the familiar framed canopy with the two car-like side doors. The air scoop was revised and moved aft. The basic armament of the P-63D was essentially the same as that of earlier Kingcobras, but the cannon in the P-63D was the M9E1 with 48 rounds.
The first P-63D (serial number 43-11718) flew early in 1945. The P-63D was the "hottest" Cobra yet to appear, with a maximum speed of 437 mph at 30,000 feet. Although the P-63D had a good performance, it was no better than the North American P-51D Mustang which was already in service. Consequently, no thought was given to any production, and only one P-63D was built. The sole P-63D was lost in a flight test accident, killing Bell test pilot Robert Borcherdt.
Specification of Bell P-63D Kingcobra:
Powerplant: One Allison V-1710-109 (E22) water-cooled engine rated at 1425 hp for take off. Performance: Maximum speed was 437 mph at 30,000 feet, service ceiling was 39,000 feet, and an altitude of 28,000 feet could be reached in 11.2 minutes. Normal range was 950 miles, and maximum ferry range was 2000 miles. Dimensions: wingspan 39 feet 2 inches, length 32 feet 8 inches, height 11 feet 2 inches, and wing area 255 square feet. Weights: 7076 pounds empty, 8740 pounds gross, and 11,100 pounds maximum loaded. Armament: One 37-mm M9E1 cannon in the propeller hub with 48 rounds, a pair of 0.50-inch machine guns in the forward fuselage synchronized to fire through the propeller arc, plus a single 0.50-inch machine gun in each of two underwing gondolas