It is probably a gross understatement to say that the designation scheme used by the Army for its pursuit aircraft during the late 1920s and the early 1930s was bizarre and inconsistent. Sometimes, an experimental change in powerplant in an existing pursuit design would call for an entirely new designation. On other occasions, it would call only for a new version letter in the existing designation. And sometimes it would call for no redesignation at all. Often, an existing pursuit airframe would be taken off the production line, experimentally fitted with a new engine, given a new designation, and then would revert back to the old designation when the engine was removed. No example typified the eccentricities of the designation system more than the wild gyrations which produced the Curtiss YP-20.
The history of the YP-20 can be said to start back with the first Army production contract for the Curtiss P-6 Hawk. The designation P-11 had been reserved by the Army for a version of the P-6 powered by the 600 hp Curtiss H-1640 Chieftain two-row twelve-cylinder air-cooled engine. Three P-11s had been ordered (Ser Nos 29-267, 29-268, and 29-374) by the Army at the same time that the original P-6 order had been issued. However, in tests with other airframes the Chieftain engine had proven itself to be completely unsatisfactory, being subject to chronic overheating problems. The Chieftain engine project was cancelled while the P-11 airframes were still on the production line. The three P-11 airframes were then used for other purposes. 29-267 and 29-368 were fitted with Conqueror engines and then delivered to the Army as standard P-6s. However, 29-374 was to have an entirely different fate.
In October 1930, Ser No 29-374 was fitted with a 650 hp Wright R- 1870-9 Cyclone radial engine. The fin and rudder were changed slightly by raising the division between the rudder balance areas and the top of the fin by half a rib space. The aircraft was redesignated YP-20, continuing the rather bizarre practice of giving new pursuit designations to existing airframes which had been experimentally fitted with different kinds of engines. A large set of wheel pants was briefly fitted to the YP-20 in the interest of attaining greater speed.
The YP-20 had a maximum speed of 187 mph at sea level and 184 mph at 5000 feet. The initial climb rate was 2600 feet/minute, and an altitude of 5600 feet could be attained in 2.3 minutes. Service ceiling was 26,700 feet. Weights were 2477 lbs empty, 3323 lbs gross. The YP-20 was armed with two 0.30-cal machine guns mounted in the upper fuselage decking, synchronized to fire through the propeller arc.
In June, 1931, the YP-20 participated unsuccessfully in a flyoff against a standard P-6, a standard Boeing P-12, and the Conqueror- powered XP-22. The high speed of the XP-22 won it a production order for 46 production examples under the designation YP-22.
After the tests were over, Ser No 29-374 was fitted with a V-1570-23 Conqueror engine and with the new nose, belly radiator, and single-leg undercarriage first tried out on the XP-22. With these changes, Ser No. 29-374 was redesignated XP-6E, and became the prototype for the famed P-6E version of the Hawk. After having proven out the P-6E concept, the aircraft was fitted with a turbosupercharger and became the P-6F.