It has been stated tongue in cheek that the P-400 is a P-40 with a Zero on its tail...
In reality, the P-400 was a P-39C/D delivered to the requirements of the RAF.
The P-39Q was a fully evolved P-39, with a more powerful engine and detail airframe and system upgrades. Beginning with the P-39Q-1, the four .30 caliber machine guns were replaced with two .50 caliber guns in under-wing pods. The Soviets, who flew the bulk of the P-39Ns and Qs built, removed the gun pods to increase over-all performance. Some P-39Qs were delivered with 4-blade propellers.
On the other hand, the P-63, although similar in appearance, was a completely new design. It was powered by a two stage, variable speed supercharger that produced a speed of at least 410 mph at 25,000 feet. It was larger in every respect, including a new laminar flow wing of much greater area. The Soviets deployed a small number of them against the Luftwaffe to gain knowledge of its combat capability. German reports indicate that those who had encounters with these "new Airacobras" were very much surprised at their performance, especially their ability to climb with late model 109s. The P-63 never saw combat with the USAAF. This was directly the result of it not having adequate range to be useful as an escort, and insufficient ordnance capability (compared to the P-47 and P-38) for use as a tactical fighter. Thus, the vast bulk went to the Soviets, who horded most of them for the anticipated upcoming battle with Japan. Some saw service with the Free French air force, but not until after Germany surrendered. These French Kingcobras did see extensive combat (ground support) in South East Asia against the Viet Min.
My regards,
Widewing