XO-19 Improved version of the XO-6 with a 450 hp (340 kW) Pratt & Whitney R-1340-3 engine, one built. YO-20 Similar to the XO-19 with a 525 hp (391 kW) Pratt & Whitney R-1690-1 engine, one built. XO-21 Similar to the XO-19 with a 600 hp (450 kW) Curtiss H-1640-1 engine, one built, later re-engined as the XO-21A.