Only one XP-13 (S/N 29-453) was built. The Viper was designed for the Curtiss H-1640-1 Chieftain engine and initially flew with it. Serious cooling problems with the twin-row in-line radial engine could not be overcome, and the XP-13 was converted to the XP-13A after an engine change (Pratt & Whitney R-1340-9) and a vertical fin redesign. The XP-13 fuselage was constructed of a metal framework with a corrugated aluminum skin giving the aircraft a very unique appearance.
XP-13 with R-1340-9 engine
TECHNICAL NOTES: Engine: Curtiss H-1640-1 Chieftain air-cooled twin-row in-line radial of 600 hp Maximum speed: 172 mph Service ceiling: 20,775 ft. Length: 23 ft. 6 in. Span: 28 ft. 0 in. Height: 8 ft. 5 in. Maximum weight: 3,256 lbs.