Curtiss O-1B serial number 27-263 was redesignated XO-18 when used as
a flying testbed for the experimental 600 hp Curtiss H-1640 Chieftain
twelve-cylinder twin-row air-cooled radial engine. The Chieftain
engine proved to be unsuitable and the XO-18 was refitted with the
standard Curtiss V-1150-5 engine and reverted to O-1B configuration.
The XA-6 was a proposed attack version of the XO-18. However, the
XA-6 was cancelled before anything could be built.