During 1928, the 600hp Curtiss H-1640 Chieftain 12- cylinder air-cooled radial appeared to show promise as a fighter power plant, and Curtiss was assigned the task of mating this engine to the P-6 airframe. A USAAC contract was placed for three aircraft which were designated P-11s. In the event, the Chieftain proved unsatisfactory and the P-11 was abandoned (together with the projected Curtiss XP-14 that had been designed around this engine). Two of the airframes that had been ordered in January 1929 were completed as P-6Ds and the third, which was fitted with a 575hp Wright R-1820-9 Cyclone, was assigned the designation YP-20 on delivery in October 1930. It was later fitted with the V-1570 engine of the XP-22, complete with cowling, the cantilever main undercarriage members and three-blade propeller of this aircraft to become the XP-6E.