Icons of Aviation History: Bell P-39 Airacobra

Originally designed to intercept high-altitude heavy bombers, the P-39 ended up performing low-altitude fighter-sweep duties for the Soviets.

RP-39Q Airacobra on display at the USAF Museum in Dayton

Throughout the 1920s and 1930s, the US military had been preparing exclusively for a defense against seaborne invasion. Since no nation had aircraft capable of crossing the vast oceans to reach the United States, not much thought was given to any capability to counter high-altitude bombers. But by the late 30s it was becoming clear from events in Europe that strategic bombers could indeed pose a serious threat in the near future.

In 1937, the US Army Air Corps finally released a “request for proposals” for an interceptor that would be capable of shooting down heavy bombers approaching at high attitude. The design would have to use the liquid-cooled inline Allison engine, it would need to reach a speed of at least 360mph, and it had to be capable of reaching an altitude of 20,000 feet in just six minutes.

In response, the Bell Aircraft Company proposed a revolutionary airplane that was different from anything else that had ever been built. The key to the P-39 Airacobra design was its gun. Since the time of World War One, fighter planes and interceptors had been armed with machine guns, sometimes of .30-caliber or, in more modern designs, .50-caliber. But in the eyes of Bell’s design team, this armament was reaching its limits and was already proving to be insufficient to bring down the kinds of big four-engined high-altitude bombers that the United States could expect to face soon. And so, they decided, they needed to upgrade the fighter’s armament to a cannon. This had already been done in some places: the Japanese Zero was armed with 20mm cannons, and so was the Messerschmitt. But Bell concluded that their new interceptor would carry a single M4 37mm cannon—twice as big—with exploding shells that were capable of  destroying a bomber with just one hit. And for maximum accuracy, this would be mounted in the nose and fire through the propeller hub.

That decision dictated the rest of the design. With the huge cannon and its ammo filling up much of the nose of the airplane, there was no room for an engine there, so the liquid cooled Allison and its radiator would have to be mounted further back, behind the cockpit. That in turn left just enough space inside the nose for two .50-caliber machine guns above the cannon, and in some later models, additional .30-caliber machine guns were mounted in the wings. The Airacobra would have devastating firepower.

The propeller shaft, meanwhile, would need to stretch all the way through the front of the plane to the propeller, passing through the cockpit underneath the pilot’s seat. This necessitated placing the cockpit higher in the fuselage, covered by a bubble canopy—which gave increased visibility for the pilot. Finally, the shifting center of gravity produced by having the heavy engine at the middle of the fuselage meant that a tail wheel would be ineffective, so the P-39 was fitted with “tricycle” landing gear at the nose and each wing. And finally, to give sufficient performance at high altitude, the Allison V-1710-17 engine would be fitted with a turbo-supercharger to suck in extra oxygen for additional power.

It was a radical design that all looked great on paper. And then it all fell apart.

When the prototype took to the air in April 1939, it hit speeds of 390mph at 20,000 feet, but the plane’s speed at lower altitude was not as great as expected, and although the P-39 had been designed for high altitudes, now the Army decided that it also wanted better performance at low altitude as well. When experts from the National Advisory Committee for Aeronautics studied the problem, they decided that the issue was with the air intake that fed oxygen into the turbo-supercharger—it was creating enough drag to affect the airplane’s performance, and if it were moved to somewhere inside the fuselage it would increase the speed by as much as 16%. This was not something that Bell wanted to hear: moving the turbo-supercharger somewhere else would require a slew of other design changes, costing time. And so, in a decision that would prove to be disastrous, Bell chose to drop the supercharger altogether. While this gave the preferred increase in speed at low altitudes, it meant that the plane’s ability to fly well at high altitude was severely crippled—a fatal flaw in a design that had been intended to attack enemy bombers.

Nevertheless, the Army ordered eighty P-39s in August 1939. A month later, Germany invaded Poland.

The initial design for the Airacobra, like most other fighters of the period, had no provisions for self-sealing gas tanks or for armor in the cockpit to protect the pilot. But after the war began, the Army decided that these things were necessary, and although it was a simple matter to add them, the unplanned-for weight also reduced the plane’s performance.

By 1940, France and England were facing the Nazi onslaught and were frantically looking for fighters—any that were available. Under Lend-Lease, the US offered them P-39 Airacobras. France surrendered before any could reach her in significant numbers, but England asked for 675 of the fighters to be shipped. When they reached the RAF 601 Squadron, the British flew four missions with them against the Luftwaffe bombers, discovered that the Airacobra’s performance at high altitudes was terrible, and refused to fly them again. Some 200 of the planes were passed on to Russia, who was also desperate for fighter planes. A handful were sent to North Africa to serve as ground-attack aircraft. The rest were sent back to the US.

After December 1941, the United States was also desperate for fighters, and it quickly took all of the P-39s intended for England and sent them to the Pacific. Many of these were armed with 20mm nose cannon instead of 37mm, a variant known as the P-400. As it happened, most of the fighting against the Japanese took place at low altitudes, where the Airacobra’s lack of a turbocharger did not hamper it, and although it was outclassed by the Zero it still held up reasonably well, and by some accounts even managed a 1:1 kill ratio with the Japanese fighters. The P-39 stayed on the Pacific frontlines until it could be replaced by P-38s and P-47s.

But it was in Russia that the Airacobra came into its own. As the US replaced its P-39s with Thunderbolts and Mustangs, it began sending Airacobras to the USSR under Lend-Lease. The Soviets quickly realized that the P-39 was not a high-altitude interceptor, but if utilized at low altitude, where most of the combat on the Eastern Front tended to be anyway, it was a very capable and heavily-armed platform for shooting down German fighters and dive bombers. (Note that the Soviets did not utilize the Airacobra in the ground-attack role, despite many postwar histories that say otherwise—they already had the Shturmovik for that, and this was part of an intentional Russian campaign to mislead the Americans about how they were using the planes.) In total, around half of all the nearly 10,000 P-39s produced during the war went to Russia, along with about three-fourths of the 5,000 later larger variant called the P-63 Kingcobra.

In Soviet hands, the Kobrushka (“Little Cobra”) was devastating against the Nazis. Of the top ten Soviet aces of the war, five scored most of their victories in P-39 Airacobras, including Aleksandr Pokryshkin with 48 victories and Grigori Rechkalov with 44. At least 28 Russian Airacobra pilots scored 15 or more.

Around 20 wartime P-39s still exist, with around a dozen of these being flight-capable. In the US, the Pima Air and Space Museum has a P-39N model on display, while the Air Force Museum in Dayton exhibits a reconnaissance version of the P-39Q.