There was a wide consensus in U.S. military circles about 1940 that U.S. pursuit fighters were substantially less capable in terms of performance than their equivalent European counterparts. Specifically, in Germany, the BF109-E (the first version to use the Daimler-Benz DB-601 engine) began production in 1938, while the original airframe was designed in 1934. In Britain, the Spitfire airframe was designed in 1935 and the airplane went into development and testing up to, and after, the start of production in 1938 or 1939. The single-engined fighters which were being developed in the U.S. at the same time were the Seversky P-35, Bell P-39, and Curtiss P-40. There was wide belief - confirmable by various performance metrics, that the U.S. fighters were inferior.
What accounted for this difference in performance? Let's assume for the moment that the discipline regarding aircraft structures was largely settled and was evolving incrementally in practice. Consequently, advances in aircraft performance would have been governed by developments in 1) aerodynamics and 2) propulsion. How did developments in these two areas progress during the war, and how did that influence aircraft production?
Keeping this in mind, one reason, for example, that immediately jumps out is that the European engines at this time were of higher performance in terms of power than similar engines then available in the U.S. The German DB-601 and British Rolls Royce Merlin engines were both superior to the American Allison engine, the only serious V12 the U.S. produced in the late 1930s. It would take until 1940 for the U.S. radial engine makers to launch high-output versions of their radial engines (e.g., super-turbocharged R-2800s) and of course the U.S. capitalized on the high-performance, liquid-cooled Merlin for use in the P-51, rather than undertake more immediate development and improvement of a similar engine by Allison.
So the question is, was the entire scope of the performance difference due to powerplants alone, or, as the war progressed, were there other salient developments in aircraft propulsion and aerodynamics that improved the performance of American fighter aircraft relative to comparable British and German fighters?