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The Curtiss XP-21 Hawk was the designation given to two P-3 Hawks when they were used to test the Pratt & Whitney Wasp Jr engine. The P-3 was a version of the standard P-1 Hawk but powered by the standard Pratt & Whitney Wasp radial engine. Six were produced - one XP-3A and five P-3As.
The XP-3A was ordered as the last P-1A. It was to have been built with a Curtiss R-1454 engine, but that failed and it was completed with the Wasp engine as the XP-3A. The first of the production P-3s also became an XP-3A (28-189) when it was used to develop the NACA cowling and came second in the Free-for-All race at the 1929 National Air Race.
Both XP-3As were then given the 300hp Pratt & Whitney R-985 Wasp Junior engine, presumably to serve as engine test beds. In this configuration they became the XP-21. The first aircraft, serial 26-300, was later given an improved R-975 War Junior and became the XP-21A while the second aircraft was given a D-12 engine and became a standard P-1F.
Engine: Pratt & Whitney R-985-1 Wasp Junior
Power: 300hp
Crew: 1