Based on the preceding Bell P-39 Airacobra, the P-63's design incorporated suggestions from P-39 pilots and was superior to its predecessor in virtually all respects.

High-altitude performance suffered dramatically as a result, and Bell proposed an experimental series to test out a variety of solutions.

At higher altitudes, when additional boost was required, a hydraulic clutch would engage the second supercharger, adding 10,000 ft (3,000 m) to the service ceiling.

A persistent complaint about the Airacobra was that its nose armament was not easily accessible for ground maintenance; to cure this problem the XP-63 airframe was fitted with larger cowling panels.

The Merlin-engined 42-78015 (as Merlins were primarily needed for the P-51 Mustang) was delivered with another Allison instead, a -93, which had a war emergency rating of 1,500 hp (1,100 kW) at sea level, making this prototype one of the fastest Kingcobras built, attaining 421 mph (678 km/h) at 24,100 ft (7,300 m).

In February 1944, the Soviet government sent a highly experienced test pilot, Andrey G. Kochetkov, and an aviation engineer, Fyodor P. Suprun, to the Bell factories to participate in the development of the first production variant, the P-63A.

Most significantly, Soviet input resulted in moving the main cannon forward, favorably changing the center of gravity, and increasing its ammunition load from 30 to 58 rounds for the A-9 variant.

[7] L-39-1 first flew 23 April 1946, demonstrating a need for extra tail surface and rear fuselage length to balance the aircraft in flight—the wing repositioning reduced empennage effectiveness and moved the center of lift aft.

[9] One of the enduring myths regarding the P-39/P-63 in Soviet use is that because of its armament, in particular the 37mm nose cannon, it excelled as a ground-attack aircraft, even a "tank buster".

The "tank buster" myth has its roots in the misunderstanding of the general wartime role of the Red Air Force and in the imprecise translation of specific Russian-Language terms...

Frequent misunderstanding ... as to the combat role of the P-39 in Soviet use is based in part on imprecise translation of the term prikrytiye sukhoputnykh voysk to "ground support".

The first version to be supplied in quantity to the Soviet Union was the P-63A-7 with a higher vertical tail, and reinforced wings and fuselage.

[13] P-63s were allegedly supplied to the Soviet Union only on the condition that they be reserved for future operations against Japanese forces, under a 1943 agreement with the US.

A Soviet pilot who served under A. I. Pokryshkin, reported that the entire 4th Guards Fighter Aviation Regiment (4 GvIAP) secretly converted to P-63s in 1944.

[citation needed] Stuka pilot Hans Rudel stated in his memoirs that, over the Courland pocket, in early 1945, he and other German pilots had often encountered "American types of aircraft, especially Airacobras, Kingcobras and Bostons"..[14] One account states they were in action at Königsberg, in Poland and in the final assault on Berlin.

Low ceilings, short missions, good radios, a sealed and warm cockpit and ruggedness contributed to their effectiveness.

In the Pacific theatre, the Kingcobras flew escort, close air support and ground attack missions.

The Soviet P-63s achieved their first air victory on 15 August 1945, when Lejtenant I. F. Miroshnichenko from 17th IAP/190 IAD, shot down a Nakajima Ki-43 Hayabusa IJAAS fighter off the coast of North Korea.

[12] On 8 October 1950, two USAF F-80Cs from the 49th Fighter Group breached the USSR's border and attacked Sukhaya Rechka airfield 19 mi (31 km) south-west of Vladivostok and 62 mi (100 km) from the Soviet-Korean border,[15] making two strafing runs before returning to their home base.

[17] In 1945, 114 later models were delivered to the French Air Force (Armée de l'Air), but they arrived too late to see service in World War II.

Fighter squadron (Groupe de Chasse) 2/6 "Travail", previously equipped with P-39 Airacobras, received their Kingcobras on 18 July at Casablanca; the pilots were surprised by the higher landing speed of their new aircraft.

[22] British engineers, like the Americans, had a growing interest during World War II in the application of laminar flow airfoils.

In an effort to learn more about the practical application of laminar flow airfoils, in 1945 the Royal Aircraft Establishment (RAE) undertook a flight test program with one of the two P-63As that the United Kingdom had received.

[23][page needed] The aircraft was equipped with a wake rake array mounted outboard, behind the wing, to allow the momentum deficit, and thus section drag, to be measured.

In the "as delivered" configuration, a profile drag was measured which was representative of the wing section with boundary layer transition at the leading edge (0% laminar flow).

[23] This gave researchers an idea of what level of wing surface quality was required to actually get the benefits of laminar flow airfoils.

Later sold to a European pilot, this P-63 was destroyed in a fatal accident in 1990[26][27] Crazy Horse Campgrounds was the most radically modified P-63 Kingcobra ever.

Larry Haven's "Race 90" clipped-wing unlimited racer had a tiny bubble canopy installed; it appeared in all silver (unpolished aluminum) finish with a white rudder and black trim.

[28] On 3 June 2001, a Bell P-63A crashed during the Biggin Hill International Air Fair in London, England, killing the pilot.

This was the second plane accident in 24 hours at Biggin Hill following the crash of a de Havilland Vampire that killed both pilots.