Combat reports coming back from Europe led the USAAC to conclude that the P-39C was inadequately armed and armored for its intended ground-support role. On September 13, 1940, 394 P-39Ds (Model 15) were ordered. The serials were 41-6722/7115. It was the first Airacobra which could be considered even remotely as being combat-ready.
On the day after the P-39D order, the initial contract for the P-39C was modified, and the 21st and subsequent machines on the P-39C order (serials 40-2991/3050) were ordered completed as P-39Ds.
The P-39D differed from the P-39C primarily in having a heavier armament. It had four wing-mounted 0.30-inch machine guns with 1000 rpg, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun, plus the 37-mm cannon (with increased ammunition capacity of 30 rounds). Bulletproof windshield panels were added, and some armor protection for the pilot was provided. Self-sealing fuel tanks were introduced, which reduced internal fuel capacity from 141.5 Imp. gall. to 100 Imp. gall. This internal fuel could be supplemented by a 72.4 Imp. gall drop tank carried on a strongpoint fitted underneath the fuselage. In place of the drop tank, a 300 lb or 600 pound bomb could be carried. A different 10 foot 5-inch Curtiss Electric propeller was fitted and the fuselage length was increased to 30 feet 2 inches. In addition, a very small dorsal fin was added just ahead of the rudder. The D model also differed from all other Airacobra variants in having a slightly smaller wing area of 213 square feet, as compared with 213.22 square feet on all other models.
The bulletproof windshield and armor protection added 245 pounds to the weight of the aircraft, causing the climb and altitude performance to suffer. The maximum speed at 15,000 feet dropped to 360 mph.
The first P-39D Airacobras entered service with the USAAC in February 1941, first with the 31th Pursuit Group (39th, 40th, and 41st Pursuit Squadrons) based at Selfridge Field, Michigan. Between February 12 and March 21, 1941 27 USAAC pilots flew three P-39Ds over 160 accelerated service test hours at Patterson Field in Osborne, Ohio. In March of 1941, Bell's test pilot Vance Breese dove an Airacobra 15,000 feet to pass the final Army Air Corps test. The 39th PS partcipated in the "Carolina Maneuvers" from September to November of 1941, which was a series of war games during which five different squadrons flew Airacobras.
The P-39D (along with the P-400) was the first to see combat in US service. There were a great many weaknesses in the Airacobra, apart from the general problem of poor high-altitude performance. Among these were the lack of gun heaters which caused the guns to freeze up and jam at altitudes over 25,000 feet, the lack of hydraulic chargers which made it difficult to charge the guns in the air, and the forward gear box just behind the propeller which had a tendency to throw oil.
Specification of Bell P-39D Airacobra:
Engine: One 1150 hp Allison V-1710-35 twelve-cylinder liquid cooled engine. Performance: Maximum speed 309 mph at sea level, 335 mph at 5000 feet, 355 mph at 10,000 feet, 368 mph at 12,000 feet, and 360 mph at 15,000 feet. An altitude of 5000 feet could be reached in 1.9 minutes. It took 5.7 minutes to reach an altitude of 15,000 feet and 9.1 minutes to reach 20,000 feet. Service ceiling was 32,100 feet. Maximum range (clean) was 600 miles at 10,000 feet at 231 mph. Range with one 145.7 Imp gal drop tank was 1100 miles at 196 mph. Weights: 5462 pounds empty, 7500 pounds gross, and 8200 pounds maximum takeoff. Dimensions: Wingspan 34 feet 0 inches, length 30 feet 2 inches, height 11 feet 10 inches, and wing area 213 square feet. Armament: One 37-mm cannon in the nose with 30 rounds. Four wing-mounted 0.30-inch machine guns with 1000 rpg, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun. One 250 lb, 325-lb, or 500-lb bomb could be carried underneath the fuselage.
40-2991/3050 Bell P-39D Airacobra 60 planes 41-6722/7115 Bell P-39D Airacobra 394 planes