Air Corps production plans had begun with Circular Proposals issued March 11, 1939, for single-engine Interceptor Pursuits, twin-engine Interceptor Pursuits, and Multiplace Fighters, with specifications calling for clones of the P-39, P-38, and YFM-1A designs. The last category was dropped from Air Corps plans, while the P-38 naturally won the twin-engine contract, but by May 10, the Materiel Division was considering less restrictive requirements for the single-engine pursuits to be delivered in 1941.
Tricycle gear, cannon, and the type of supercharging became optional on a new specification dated June 24, which enabled Curtiss, Republic and Vultee to enter the contract competition. Republic won first place with an order for 80 P-44s with Wasp engines, while 80 Bell Model 4-F fighters, designated P-45, would have the new 1,150-hp V-1710-35 Allisons promised for delivery in 1941. Contracts for the XP-46 and XP-47 designs were also awarded by this competition.
Bell’s contract was approved October 12, 1939, but this model was redesignated P-39C. The P-39C specification dated February 14, 1940, estimated top speed as 343 mph at sea level and 400 mph at 15,000 feet, with a 39,000 feet service ceiling, based on 1,150 hp expected at 15,000 feet.
Bell Aircraft obtained a Foreign Sales Agreement from the Army on April 4 that allowed them to sell Airacobras to the Allies in exchange for designing a new fighter, the Model 16, which began the XP-52 project. Always an aggressive salesman, Larry Bell hurried to convince the joint Anglo-French Purchasing Commission to buy his “P-400”. Unlike the Army, which then paid only after its planes were delivered, the Allies paid two million of the nine million dollar contract in advance; money his company badly needed.
This contract, made for France on April 13, 1940, and approved May 8, specified a 20-mm Hispano gun as the cannon armament and added wing guns, armor, and leak proof tanks. Despite the increase in weight, Bell guaranteed a top speed of 383 mph at 15,000 feet. This contract was taken over in June by the RAF, which by June 20, 1941, increased the number from 170 to 675 aircraft and planned to make the P-400 the first single-seat American to enter Fighter Command itself, rather than serving overseas like the Buffalo and Kittyhawk.
The full set of weapons mounted in the nose included two .30-caliber guns with 1,000 rounds, two .50-caliber guns with 400 rounds and a 37-mm cannon with 15 rounds. This YP-39 gun’s first air test firing of 500 rounds was from November 7 to 24 over Lake Ontario, and established that cannon as Bell’s trademark weapon.
All Army P-39s were ordered with that cannon, which, Capt. Kelsey explained in an October 25, 1940, memo, was preferable at lower altitudes against heavily armored aircraft and tanks. Since the Lockheed P-38E was expected to operate “at high altitudes against lightly armored planes”, the 20-mm gun was chosen for the Lightning.
The fourth YP-39 was delivered to Wright Field on December 2, 1940, and the next YP-39s went to Chanute, Patterson, and Lowry Fields and to NACA for tests. The last five, delivered December 16, 1940, went to Selfridge Field.
By September 13, 1940, the Army had added contracts for 623 P-39D Airacobras, which was the Bell Model 15 with wing guns and Hewitt Rubber Co. self-sealing fuel bags, and ordered that these additions be made to the last 60 aircraft on the previous contract. To expedite production, it was soon agreed to standardize most P-39D and P-400 features except for the latter’s 20-mm gun and British-installed systems like oxygen and radio.
The first 20 P-39C production aircraft were completed from January 16 to April 7, 1941, with 1,150-hp V-1710-35 engines, camouflage finish, and the same five guns as the YP-39. Selfridge Field got them for the 31st Pursuit Group, although three were shipped to Britain on June 26. Thirty more sets of leak proof tanks were ordered on March 8, 1941, for later installation on YP-39 and P-39C models.
The P-39D and export P-400 came off the assembly lines at the same time, both with the V-1710-35 engine, self-sealing tanks, four .30-caliber wing guns with 4,000 rounds, and two synchronized .50s with 400 rounds in the nose. The P-39D had a 37-mm M-9 gun with 30 rounds, 191 pounds of armor, and 66 pounds of armor-glass. Since tank protection reduced internal fuel capacity from 170 to 120 gallons, a February 1941 order added provisions for a 75-gallon belly drop tank, or a 600-pound bomb, on the P-39D.
The P-400 used the more rapid-firing 20-mm HS gun with a 60-round drum, 231 pounds of armor and 60 pounds of armor-glass, the heaviest protection of any American fighter, as well as the 240-pound leak proof fuel bags and a 110-pound radio.
The first P-39D, priced at $41,479, was delivered February 3, 1941, but remained at the factory until December, while regular deliveries to Selfridge Field began May 5. Bell completed 429 P-39D models in 1941, too quickly for the available guns or Curtiss Electric propellers; only 390 37-mm aircraft guns in total were completed that year. The aircraft were sometimes flown to an Army base where the propeller was removed, shipped back to Bell, and reused to fly away another plane.
Aeroproducts hydromatic propellers were substituted on 229 P-39Fs, delivered beginning in December 1941, and 25 other ships on this contract were designated P-39J with Allison V-1710-59 engines rated at 1,100 hp at 15,200 feet with automatic boost control. All these aircraft were known as Model 15 on company records and had similar characteristics.
The British Model 14 planes were called Airacobra I by RAF and P-400 in U.S. records. When the second was tested in April 1941 in light and highly polished condition, Larry Bell wrote the Army that it had achieved a 392-mph top speed at 14,400 feet and a 35,000-foot service ceiling. He did not mention the limited gross weight, which omitted guns and armor. In August, a standard P-400 was claimed to have a 371-mph top speed at 14,090 feet and a 34,800-foot service ceiling.
But after the P-400 reached Britain on July 30, 1941, it was tested at a full weight of 7,830 pounds, and strict RAF standards listed the top speed as a disappointing 355 mph at 13,000 feet with a 29,000-foot service ceiling. This loss of speed could not be explained by the American support group, and it became apparent that the company had greatly exaggerated its product’s virtues. Compared to the Spitfire VB, the Airacobra I was faster at low levels, but decidedly slower in climb, and increasingly slower at altitudes over 14,000 feet.