One of the most innovative designs of World War II, the Airacobra featured most notably a mid-engine driving the front-mounted tractor prop with a long drive shaft. It was also noteworthy for being a tricycle gear design in an era of taildragger fighters. Bell, now known for its helicopters, built nearly 10,000 P-39s, a surprising fact as the type is not as well known as other rival Allied fighters like the Mustang and Corsair. The P-39 was very fast at low altitude, but its insufficiently powerful turbocharger limited its high-altitude capability, often relegating it to ground support duty. The Soviets loved the P-39, and used it to down, along with other Allied operators, more enemy planes than any other American made fighter.