The whole layout flowed from one weapon: the 37 mm T9/M4 cannon. To fire a gun that large straight down the aircraft’s centreline and through the spinning propeller, Bell needed the nose clear of any engine. So the Allison V-1710 was mounted amidships, over the wing and behind the cockpit, driving the propeller through a roughly 3-metre (10-foot) extension shaft that passed beneath the pilot’s seat to a gearbox in the nose. The mid-fuselage engine also put the aircraft’s mass near its centre of gravity, which helped Bell adopt tricycle landing gear — a nose-wheel arrangement still unusual for a fighter in 1939 — and the distinctive automobile-style doors with wind-down windows. The trade was cooling and centre-of-gravity complexity, and an early reputation for tumbling if mishandled; the payoff was a hard-hitting, easy-to-service, good-visibility fighter.

The single-stage Allison V-1710 made good power at low and medium altitude, but its performance faded higher up unless boosted by a turbo-supercharger. The XP-39 prototype had one; the production P-39 did not. After NACA wind-tunnel work aimed at cutting drag, the turbo installation — bulky and aerodynamically awkward — was dropped. The result was a fighter that ran out of breath above roughly 15,000 feet, precisely the band where the air wars over Britain, Germany and the high-flying Pacific were decided. Historians still debate whether keeping the turbo would have made the Airacobra a front-rank interceptor; what is certain is that its deletion pushed the type toward the low-altitude ground-attack and Eastern-Front roles where it excelled.

Soviet pilots called the P-39 the Kobra, and it became one of the most valued Lend-Lease aircraft they received. Most Eastern-Front combat happened below 5,000 metres, where the Airacobra’s lack of a supercharger was irrelevant and its heavy 37 mm cannon, four machine guns, rugged airframe and superb forward view were decisive. Aleksandr Pokryshkin, the second-highest-scoring Allied ace of the war, is commonly credited with scoring most of his 59 victories in the P-39, and refined his “altitude–speed–manoeuvre–fire” tactics in it. Fellow ace Grigori Rechkalov was also among the top Airacobra scorers. No other American fighter was flown to greater effect by a foreign air force.

As a mass-produced wartime fighter, the Airacobra was relatively cheap. Period figures put its flyaway cost in the region of $50,000 in early-1940s dollars — an order-of-magnitude figure that varied by block and year, not a fixed price. No meaningful cost-per-flight-hour figure exists for the type in service; the only modern operating costs come from the handful of airworthy warbirds, whose owners typically cite piston-warbird running costs in the low thousands of dollars per hour. Treat any precise dollar figure attached to a P-39 as an estimate.