While the YP-39 and the P-39C were being test-flown, Bell Aircraft began work on an export version of the Airacobra known as the Bell Model 14. It was to be powered by a 1150 hp Allison V-1710-E4 engine which had twelve exhaust stacks on each side rather than the usual six. France was sufficiently interested that they ordered 200 Model 14s on October 8, 1939.
All of the media hype surrounding the spectacular performance of the XP-39 prototype had caught the attention of the British Direct Purchase Commission which had visited the USA in 1940 in search of combat aircraft. Seduced by promises of 400 mph top speed, a tricycle undercarriage, heavy cannon armament, and high climb rates, the British ordered 675 examples of the Airacobra. Unfortunately, Bell's glossy advertising brochures did not distinguish between the performance of a lightly-loaded, unarmed, highly-polished experimental prototype and a production fighter heavily-loaded with military equipment and armament, and the British were to rue the day that they ever looked at an Airacobra.
In 1940, the British were desperate for combat aircraft and were willing to consider just about anything that had wings, irrespective of how poor its performance might be. Consequently, when Bell submitted specifications to the British Direct Purchase Commission for a fighter with a top speed of 400 mph, a ceiling of 36,000 feet, and a range of 1000 miles, the Commission literally salivated on the spot and ordered 675 Bell Model 14s sight unseen on April 13, 1940.
The RAF model was at first named Caribou, but the American name of Airacobra was adopted in July 1941. The British Airacobra was virtually identical to the American P-39D, but the slower-firing 37-mm cannon was replaced with the faster-firing and more reliable Hispano 20-mm cannon with 60 rounds. Two 0.50-inch machine guns were mounted in the fuselage, and four 0.30-inch machine guns were mounted in the wings. The engine of the Model 14 was the 1150 hp Allison V-1710-E4 (-35). The British serials of the Airacobras were AH570/AH739 (170 planes), AP264/AP384 (121 planes), BW100/BW183 (84 planes), and BX135/BX434 (300 planes).
Bell began test flying the first Model 14 Airacobra I in April 1941. It carried the British serial number AH570. Tested on the second British Airacobra (AH571) was a revised rudder of more angular shape and less area. Although the aircraft was delivered to England in this form, this rudder was not adopted as standard. A very small dorsal fin just ahead of the rudder became a standard feature of the RAF Airacobras and was also a distinguishing feature of the American P-39D and subsequent versions.
President Roosevelt signed the Lend-Lease Act on March 11, 1941, permitting large quantities of war supplies to be transferred to the Allies. Among the early requisitions under Lend-Lease were three P-39Cs intended for "war tests" plus a batch of 150 Airacobra IAs--the A suffix being used to distinguish between Lend-Lease and Direct Purchase machines, which were otherwise identical. In the event, only the three P-39Cs were ever delivered as British machines, and were assigned the serials DS173/DS175 (USAAF werials 40-2981, -2983, -2984). The P-39Cs could be distinguished from the British Airacobras by the four machine guns in the nose and the lack of wing guns.
The first of these P-39Cs actually arrived at RAF Colerne on July 3, 1941, followed by the other two the next day. It made its first test flight in England on July 6. However, during trials at Duxford, the performance proved disappointing. Although the test pilots praised the general ease of handling of the aircraft, the maximum speed was a shocking 33 mph lower than that anticipated. The fighter proved to be definitely inferior to the Hurricane and Spitfire in climb rate and ceiling, and the 750-yard takeoff run of the Airacobra excluded its operation from some smaller fighter airfields. There was universal shock and dismay among the RAF personnel. What had gone wrong? Bell Aircraft executives later sheepishly admitted that their performance figures had been based on the unarmed and unequipped XP-39 prototype, which weighed a ton less than the armed and equipped P-39C.
The first British-purchased Airacobras began arriving at Colerne before the end of July, joining the three P-39Cs already there. Deliveries of the Airacobra to Britain had to be made by sea, since the Airacobra lacked the range to make the Atlantic crossing. By the end of September, eleven machines had been received. No. 601 "County of London" Squadron was selected to be the first Fighter Command squadron to equip with the Airacobra.
No. 601 Squadron pilots found numerous flaws and weaknesses during their initial work-up with the the Airacobra. Some of them were a question of improving operational efficiency and pilot comfort, but others were considered essential to make the aircraft operational. Numerous modifications were made in the field in an attempt to make the aircraft suitable for combat. A master valve was introduced to allow oxygen to be turned on from the cockpit. The gunsight was modified to improve forward visibility. Changes to the ammunition tanks for the wing guns were made. Modifications were made to the cockpit harness release in order to simplify the operation. The IFF (Identification, Friend or Foe) set was removed from behind the pilot, where it obstructed aft view. A throttle control quadrant friction damper was introduced.
The Air Fighting Development Unit received a British Airacobra I on July 30. They subjected it to tests and completed their report on September 22. They found the aircraft to be pleasant to fly and easy to takeoff and land. Controls were well balanced and although heavier than those of the Spitfire at normal speeds, did not increase appreciably in weight at high speeds as they did in the Spitfire. It was difficult to hold the aircraft in a dive at high speeds unless the aircraft was trimmed nose-heavy. During a turn, the Airacobra would give ample warning of a high-speed stall by severe vibration of the whole airframe. Handling in formation and formation attacks was good, although deceleration was poor because of the plane's aerodynamic cleanliness. Take-offs and landings in close formation were not considered safe, since there was considerable difficulty in bringing the aircraft back to its original path after a swing.
The Airacobra I was powered by an Allison V-1710-E4 twelve-cylinder V in-line engine rated at 1150 hp for takeoff. Weights were 5462 pounds empty and 7845 pounds normal gross. Maximum speeds were 326 mph at 6000 feet, 343 mph at 10,000 feet, 355 mph at 13, 000 feet, 341 mph at 20,000 feet. Initial climb rate was 2040 feet per minute. With an internal fuel capacity of 100 Imp gal the Airacobra had an endurance of 1 hour 20 minutes at maximum continuous cruising speed at 6000 feet, 1 hour 5 minutes at 12,000 feet, and 1 hour 35 minutes at 20,000 feet. The true airspeeds at these altitudes were 287 mph, 327 mph, and 308 mph, respectively. Under most economical cruise conditions, the endurance increased to 3 hours 20 minutes, the relevant speeds being 183 mph at 6000 feet, 217 mph at 12,000 feet, and 215 mph at 20,000 feet. Under maximum continuous climb conditions, it took 15 minutes to reach 20,000 feet. The operational ceiling was considered to be about 24,000 feet, although there was a marked decrease in performance above 20,000 feet. At the Airacobra's rated altitude of 13,000 feet, it was 18 mph faster than the Spitfire VB. However, the speed fell off rapidly above that height, and the two planes were almost exactly matched at 15,000 feet. At 20,000 feet, the Spitfire VB was 35 mph faster and at 24,000 feet it was 55 mph faster. The ground run of the Airacobra during takeoff was 2250 feet, as compared with 1470 feet for the Hurricane II and 1590 feet for the Spitfire V.
The AFDU also did some comparative dog-fighting tests with the Airacobra against a Spitfire VB and a captured Messerschmitt BF 109E. The Airacobra and the Bf 109E carried out mock dog-fighting at 6000 feet and 15,000 feet. The Bf 109E had a height advantage of 1000 feet in each case. The Bf 109, using the normal German fighter tactics of diving and zooming, could usually only get in a fleeting shot. The Bf 109 could not compete with the Airacobra in a turn, and if the Bf 109 were behind the Airacobra at the start, the latter could usually shake him off and get in a burst before two complete turns were completed. If the Bf 109 were to dive on the Airacobra from above and continue the dive down to ground level after a short burst of fire, it was found that the Airacobra could follow and catch up to the Bf 109 after a dive of over 4000 feet. When fighting the Bf 109E below 20,000 feet, the Airacobra was superior on the same level and in a dive.
A similar trial was carried out against a Spitfire V. Although the Airacobra was faster than the Spitfire up to 15,000 feet, it was outclimbed and out-turned by the Spitfire. Unless it had a height advantage, the Airacobra could not compete with the Spitfire. If on the same level or below, at heights up to about 15,000 feet, the Airacobra would have to rely on its superior level and diving speeds and its ability to take negative "G" without the engine cutting out. Above 15,000 feet, the Airacobra lost its advantage in level speed.
The Airacobra was considered to be very suitable for low altitude operations because of the excellent view and controllability, and it was fully maneuverable at speeds above 160 mph. It was not difficult to fly at night, but the exhaust flames could be seen by another aircraft flying three miles to the rear. The flash from the nose guns was blinding, and could cause the pilot to lose not only his target but also his night vision. Firing of the nose guns caused the buildup of carbon monoxide contamination in the cockpit, and this could reach a lethal level very quickly. The guns were fairly inaccessible, and maintenance was troublesome.
By the end of September, No. 601 Squadron had received permission to take its Airacobras into action. On October 9, two Airacobras took off from RAF Manston and flew across the Channel on a "rhubarb"--a code name for a small-scale raid by fighters against targets of opportunity. On this raid, they shot up an enemy trawler near Gravelines. The next day two Airacobras visited the same area, but found no targets. On October 11, two aircraft flew to Gravelines and Calais and hit some enemy barges and then three Airacobras flew to Ostend, but no targets were found.
After these four missions, the RAF Airacobras were taken off operations because of difficulties encountered with the compass. The compass was too close to the guns in the nose, and when the guns were fired, the compass got thrown out of alignment. Deviations of anything from 7 degrees to 165 degrees were recorded. Without a reliable compass, pilots tend to get themselves lost. In December of 1941, the Airacobra was officially withdrawn from operational service with the RAF.
In spite of the problems with the compass and the need for flame dampers for the exhaust and flash suppressors for the nose guns, the RAF concluded that the Airacobra would make an excellent day fighter at altitudes below 20,000 feet and was well suited for the ground-attack role. However, before these plans could be implemented, a decision was made to divert the bulk of the British Airacobra contract to Russia.
By the time this decision was made, production of British-contract Airacobras had reached four a day at Bell's Buffalo plant. The initial contract for 170 planes (RAF serials AH570 thru AH739) had been completed before the end of September, and all but six of these planes had actually been shipped to Britain. However, many of them remained in their crates and were shipped directly to the Soviet Union without being opened. Somewhere between 80 and 100 Airacobras were assembled and flown in Britain by the end of 1941. They were gathered at maintenance units for final modification before being re-crated and shipped to the Soviet Union during 1942. In all, the Soviet Union received 212 of the British Airacobras (some of them shipped direct from the USA), but 49 more were lost at sea en route.
No 601 Squadron relinquished its 13 Airacobras in March of 1942 in favor of Spitfires. One Airacobra was fitted with an arrester hook and was used for deck landing trials at the RAE at Farnborough.
After Pearl Harbor, the USA found itself in desperate need of aircraft to stem the Japanese onslaught in the Pacific. Consequently, nearly 200 of the British direct-purchase Airacobras still in the USA were promptly requisitioned by the USAAC. Although they were similar to the USAAC's P-39Ds, they were not identical and were known by the USAAC under the non-standard designation of P-400. The P-400 designation had, in fact, been associated with the British Airacobras for contractual purposes as early as August 1941. The USAAC P-400s retained their original British serial numbers and their three-color camouflage paint. Most of these planes were used for training stateside, but some of them were rushed to the Southwest Pacific in an attempt to stem the onrushing Japanese advance.