The fourth Curtiss P-36A (serial number 38-4) was used by the US Army and NACA for aerodynamic research in an attempt to overcome the aerodynamic drag penalty inherent in large-diameter air-cooled radial engines as compared to narrower liquid-cooled Vee-type engines. The aircraft was given the company designation of Model 75S, and the USAAC assigned it the designation of XP-42.
As initially delivered in March of 1939, the XP-42 had a special 1050 hp Pratt and Whitney R-1830-31 radial engiine fitted with a long extension fitted to the propeller shaft and nose casing which permitted the use of a streamlined nose with a large propeller spinner. The intake for cooling air was located under the engine, and the intake for carburetor air was located above the engine.
The initial configuration of the XP-42 suffered from serious overheating problems and from vibrations of the propeller shaft. Attempts to cure these problems resulted in no less than twelve different cowling designs being tested on the XP-42. Various types of cowl flaps were fitted, and short-nose high- and low- inlet velocity cowlings were tried with and without fans. The nose was progressively shortened until the airplane gradually once again resembled a P-36A.
The XP-42 was entered by Curtiss in the 1939 USAAC fighter competition. Although the XP-42 was faster than the P-36A, it was slower than the XP-40. Consequently, the XP-42 lost out to the XP-40 for production orders.
The XP-42 was finally scrapped in January 1947.