The P-3 was an attempt to adapt the Curtiss P-1 Hawk airframe to a radial engine. Like the Navy, the USAAC was initially undecided as to whether it preferred radial or in-line engines for its fighters. The radial engine had been successfully applied to the Boeing P-12, and the Army wanted to determine if the same could be done for the Curtiss Hawk.
The designation XP-3 had originally been reserved for P-1A Ser No. 26-300 which was to be fitted with the new experimental 390 hp Curtiss R-1454 air-cooled radial engine. However, this engine had already been tested in other aircraft and had been found to be unsatisfactory. Consequently, the XP-3 designation was cancelled before this new engine could be installed.
In October 1927, P-1A Ser No 26-300 was fitted instead with the new 410 hp Pratt and Whitney R-1340-1 Wasp air-cooled radial engine. This aircraft was redesignated XP-3A No. 1 (factory designation Model 34N). This aircraft was originally delivered to the Army uncowled, but the plane was used by the Army to test some early NACA cowling designs for radial engines. It was assigned test number XP-451.
The lighter engine installation gave the XP-3A an improved climb and ceiling performance as compared with the P-1 series. The Army was sufficiently impressed that they ordered 5 P-3As on December 27, 1927. Serials were 28-189/193. Deliveries began in October 1928. Power was provided by the 450 hp Pratt and Whitney R-1340-3 (SR-1340B). Maximum speed was 171 mph at sea level, 168 mph at 5000 feet. Initial climb rate was 1742 feet per minute. Service ceiling was 23,000 feet. Weights were 2024 pounds empty, 2730 pounds gross. Range was 342 miles. Armament was two 0.30-cal machine guns mounted in the upper fuselage, synchronized to fire through the propeller arc. The P-3As were originally delivered to the Army completely uncowled, but narrow Townend rings were soon added. These rings did little to increase the speed over that of the original uncowled XP-3A.
The designation of XP-3A 26-300 was changed to XP-21 when it was used to test-fly the 300-hp R-985-1 Wasp Junior in December 1930. This new designation was used to identify a particular test configuration and not a new prototype. However, this engine had only half the power of service fighters of the period, and the project was not successful. A later engine change to the 300 hp Pratt and Whitney R-975 radial changed the designation of 26-300 to XP-21A.
The designation XP-3A No. 2 was applied to a production P-3A (Ser No 28-189) which was used for test work for the development of a NACA cowling with fuselage faired to match the cowl. The XP-3A No. 2 was fitted with a tight cowling and large spinner, and was entered in the 1929 National Air Race Free-for-All. It came in second at 186 mph, when the Army raced against civilians for the last time. This airplane was later redesignated XP-21 No. 2 when it was fitted with a Pratt and Whitney R-985-1 Wasp Junior engine of 300 hp. 28-189 was later brought up to standards of P-1F following a change to D-12 (V- 1150-3) engine.
Tests with the P-3 and the XP-21 failed to convince the Army of any intrinsic superiority of the radial engine for the Hawk, and it was decided that the Hawk line of Army fighters was to stick with the liquid-cooled engine.