The P-39 sent to Britain under Lend-Lease was factory Model 14A. 494 with USAAF designations of P-39D-1 and -2 were ordered.

    In July 1941 the RAF received three P-39Cs with an Allison delivering 1,150hp up to 12,000', after which power fell off sharply. Observed top speed in RAF tests was 359 mph, slower than they had been led to believe. Shortly after RAF received first Airacobra Is (which were later redesignated P-400 by USAAF) with self-sealing tanks, external drop tank, 20mm nose cannon, among other things. RAF flight tests demonstrated top speed of 355 mph, faster than Spitfire VB at 15,000' but with a slower rate of climb.

    RAF 601 Sqn was equipped with Airacobras in Sept 1941 and they were in service about three months in numerous cross-channel missions to strafe ground targets and river barges. They were removed from service at the end of the year with performance deemed unsatisfactory and range too short.

    RAF accepted only a total of 80 Airacobras, the rest of the order went to USSR, and to USAAF, who redesignated its 179 planes P-400 and assigned them along with P-39s to the 8th, 35th, and 67th Fighter Groups in the Southwest Pacific. By 30 June, 20 P-400s had been destroyed in combat. In July, P-400s were only able to intercept four of nine Japanese bombing raids because they climbed too slowly above 12,000'. Thereafter they undertook strafing attacks on Japanese landing craft, where eight P-400s were lost in ten days, but invasion forces were severely damaged.

    The 67th's 14 P-400s went to Guadalcanal in late Aug 1942, where in combat four were destroyed and six damaged. They were then switched to ground attacks, where they performed admirably, and their tricycle gear enabled them to take off from fields so muddy that other types could not. P-400s played a key role in beating off the enemy attack on Bloody Ridge (aka Edson's Ridge, Lunga Ridge) on 14 Sep 1942, where strafing attacks by three planes—the entire available force—decimated Japanese troop concentrations. They continued yeoman work during the rest of the Guadalcanal campaign, sometimes flying as many as 11 sorties per plane per day. P-39s did solid ground attack and anti-shipping work in Solomons, becoming expert boat-busters. The P-400s were gradually replaced by P-39s in the Pacific, but were still being sent from England to Africa in early 1943 (81st FG, 350th FG). 52 P-400s were in Africa at end of Jan 1943, carrying out low-level strafing attacks with considerable success and low losses.

In the Southwest Pacific at the end of July, the 5th AF still had 30 P-400s, averaging 300 combat flying hours each. P-39s and P-400s were considered useless above 17,000' by 5th AF brass and unequal to the heavy demands being made on fighter forces in the theater. They requested P-38s, but were offered P-63s, which they countered with a request for P-47s—they ended up with P-40s.

    In mid-summer 1942 the Soviets requested the USAAF stop sending P-40s and asked for as many P-39s as possible, which they considered a great fighter. The elite Soviet Guards Fighter Regiments were equipped with P-39s, and its Commander in the Stalingrad area said the only criticism he had of the P-39 was that he didn't have enough of them! In Oct 1944, they began replacing P-39Qs with Soviet-made fighters.

    In the Mediterranean theater, the 350th F/G worked their P-39s effectively for ground attack, as did the 352nd F/G, based in Italy in Feb 1944. In June, the 12th AF got P-39Qs, but two months later they were replaced by P-47s. The Italian Air Force, impressed into the Allies air forces and equipped with P-39Ns, flew first anti-German combat missions on 8 Sep 1944, with top cover provided by Macchi fighters. They flew their final P-39 sorties of the war in Apr 1945, strafing and dive-bombing German positions relentlessly. At war's end, they had 83 P-39s, which remained on their active roster until 1951!

    In the Pacific in Mar 1944, P-39s attacked Rabaul, surviving extremely heavy flak, and continued with their ground attack role. The last P-39 missions flown by USAAF were in New Guinea by 347th FG in August.

When analyzing the effectiveness of P-400 and P-39, it should be kept in mind that during much of its fighting in the Pacific and on the Eastern Front it was consistently outnumbered and had to counter an aggressive foe on the offensive. Under those conditions a good ground-attack plane was vital, and once the ability of the Airacobra in this role were realized, it performed admirably. When the RAF put the Airacobra into service, they didn't really have a pressing need for such a fighter, and only carried out cross-channel nuisance raids by a handful of aircraft, which found themselves at the mercy of the entire German defense system, at a time when even Spitfires making cross-channel ventures had a rough time, and when the RAF had no desire to risk pilots on such non-vital missions.

    In the Southwest Pacific in the early days, the AAF desperately needed a fast-climbing interceptor to tackle Japanese bombing raids. The P-400/P-39 simply could not fulfill this role, but the AAF also needed a ground-attack aircraft to help cope with the Japanese invasions. In this role the P-39 was perfect, for its only major limitation—a significant one—was its range. Even external tanks added little to its 120-gallon internal fuel capacity. The Soviet Union also had a desperate need for such a ground-attacker, especially one that could double as an effective fighter in the low-level air-to-air brawls that characterized the Eastern Front. This described the P-39.

    The P-39 also did well in the Mediterranean as a ground attack plane, but there, surrounded by everything from Beaufighters to A-36s, the P-39's virtues were not distinctive—it was just another one of the pack. It's interesting to note that the P-40 was considered more vulnerable to damage in ground attacks than the P-39—one reason the Soviets aske for P-40s to be replaced by P-39s. The P-40 was noted for being able to take punishment in air-to-air combat. With all the machinery up front, there was little to be damaged in a rear attack, whereas the P-39, with all of its plumbing in back, was very vulnerable to a stern attack.

    Maybe it was a good thing that P-400/P-39 pilots couldn't get up high where all the fighter action was in the Pacific, and maybe engine placement explains why the P-39 was considered an "easy kill" by Luftwaffe pilots, not because it was a poor performer but because it was so vulnerable to the common stern attack. Southwest Pacific debriefs noted that P-39s shot down in air-to-air battles were seen not streaming fire, but coolant—a sure sign of plumbing punctures.