The Curtiss XP-21 is yet another example of the infuriatingly inconsistent designation system used by the USAAC during the 1920s and early 1930s.
The history of the XP-21 begins with the Curtiss P-3 pursuit. The P-3 was an attempt to adapt the Curtiss P-1 single-seat biplane fighter to a radial engine. The last P-1A of the series (Ser. No 26-300) had been modified as the XP-3A with the replacement of the original liquid-cooled Curtiss D-12 by a 410 hp Pratt and Whitney R-1340-1 air-cooled radial engine. This airplane flew for the first time in April 1928, and became the prototype for the P-3A series, of which only five examples were built (Ser Nos 28-189/193).
In 1930, XP-3A Ser No 26-300 and P-3A Ser No 28-189 were used as flying test-beds for the new 300 hp Pratt and Whitney R-985 Wasp Junior engine. These planes were then redesignated XP-21. This new designation was intended to identify a particular test configuration and was not intended to indicate a new prototype. The first flight of an XP-21 took place in December 1930. Tests did not convince the Army that there was any intrinsic superiority of the radial engine as a powerplant for the Hawk. Consequently, XP-21 Ser No 28-189 was later fitted with a D-12 engine and became a standard P-1F. However, XP-21 Ser No 26-300 continued on as a testbed and became XP-21A when it was fitted with an improved 300 hp R-975 Wasp Junior engine.