The Curtiss P-11 Hawk was an attempt to adapt the P-6 airframe to the new 600 hp Curtiss H-1640 Chieftain two-row, 12-cylinder air-cooled engine.
Three P-6 airframes (29-367, 368, and 374) were to be reserved for the P-11 project. However, a similar conversion of an existing airframe to the Chieftain engine had already progressed somewhat further along--a standard production Curtiss O-1B Falcon two-seat observation plane converted as a flying test bed. When fitted with the new Chieftain engine, the O- 1B was redesignated XO-18. Flight tests with the XO-18 showed that the new Chieftain engine was subject to chronic overheating problems. It was thought that the problems with the Chieftain were insoluble, and any further work on this engine was abandoned. The XO-18 flying testbed was converted back into standard O-1B configuration with a Conqueror engine.
With the failure of the Chieftain engine, any thoughts of proceeding further with the P-11 project were abandoned.
Ser Nos 29-367 and 268 were subsequently completed to P-6 standards (later converted to P-6D) with the installation of Conqueror engines. Ser No 29-374 was to have a completely different history. It later became the YP-20 when a 650 hp Wright R-1860-9 Cyclone radial engine was fitted. The fin and rudder of the YP-20 were changed slightly by raising the division between the rudder balance area and the top of the fin by half a rib space. The Cyclone radial engine installation in the YP-20 proved to be unsatisfactory, and this engine was replaced with the 700 hp V-1570-23 Conqueror liquid-cooled engine, mounted with an new horizontal tail, and given the single-strut undercarriage first tried out on the XP-22. A tailwheel was used in place of the tail skid. The YP-20 was then redesignated XP-6E, to become the prototype of the famed P-6E variant of the Hawk pursuit.