The P-3 was an attempt to adapt the Curtiss P-1 Hawk airframe to a radial engine. Like the Navy, the AAC was initially undecided if radial or in-lines were more preferable for its fighters. The radial had proven successful in the Boeing P-12, and the Army wanted to see if that would apply for the Hawk.

    The designation XP-3 had originally been reserved for P-1A [26-300], which was to try the experimental 390hp Curtiss R-1454 radial; however, that engine had already been tested in other aircraft and found to be unsatisfactory. Consequently, the XP-3 designation was cancelled before the engine could be installed. In Oct 1927 [26-300] was fitted instead with the new 410hp P&W R-1340-1 Wasp radial and redesignated XP-3A (factory Model 34N). Delivered uncowled, the plane would test some early NACA cowlings designed for radial engines.

    The lighter engine installation gave XP-3A an improved climb and ceiling performance compared with the P-1 series. The Army was sufficiently impressed to order five P-3As on Dec 27, 1927 [28-189/193] and deliveries began in Oct 1928 with a 450hp R-1340-3 (aka SR-1340B) installed. Top speeds were 171 mph at sea level, 168 at 5000'. Initial climb was 1742 fpm; service ceiling 23,000'. Weights were 2024# empty, 2730# gross. Armament was two synchronized .30 machine guns mounted in the upper fuselage. The P-3As were originally delivered to the Army completely uncowled,. Narrow Townend rings were soon added, but did little to increase the speed.

    When used to test-fly the 300hp R-985-1 Wasp Junior in Dec 1930, its designation was changed to XP-21, identifying it as a particular test configuration and not a new prototype. However, the engine had only half the power of service fighters of the period and the project was not successful. A later engine change to the 300hp P&W R-975 radial again changed the designation to XP-21A.

The second XP-3A was a production P-3A [28-189] used to test a NACA cowling with fuselage faired to match the cowl. Fitted with a tight cowl and large spinner, it was entered in the 1929 National Air Race Free-for-All—the last time the Army raced against civilians—and came in second at 186 mph. It was redesignated as the second XP-21 when it got a 300hp P&W R-985-1 Wasp Junior, then was later brought up to P-1F standards with a change to Curtiss D-12 (V-1150-3).

    Tests with the P-3 and the XP-21 failed to convince the Army of any intrinsic superiority of the ar-cooled radial for the Hawk, and the decision was made to stick with the liquid-cooled engine.