Thomas-Morse XP-13 Viper
The XP-13 Viper prototype was one of several B. Douglas Thomas designs built in hopes of a production contract from the Army, following the successful Thomas-Morse MB-3 of 1919. Financed by the company, and named the "Viper", it was officially purchased by the Army in June 1929 and designated "XP-13".
The Viper was delivered to the USAAC for evaluation in early 1929. The aircraft was tested at Wright Field in June 1929 as P-559, then purchased by the Army and designated XP-13. The serial number was 29-453. Performance was satisfactory, but the Chieftain suffered with insurmountable cooling problems. Similar problems had been encountered with Curtiss-built fighters powered by this engine.
The XP-13 fuselage had a corrugated aluminum skin built over a metal frame. The wing was of wooden construction with fabric covering, but the ailerons were made of corrugated metal sheet. Tail surfaces were of metal and fabric, but the control surfaces were covered with corrugated sheet metal. Designed to use the 600 hp Curtiss H-1640-1 Chieftain engine, (a 12-cylinder two-row air-cooled radial with the rear cylinders directly behind the front cylinders rather than staggered as normal in a two-row radial) incorporating a system of baffles to direct cooling air over the engine, the engine would not stay cool enough. In September 1930 it was replaced with a Pratt & Whitney SR1340C Wasp of 450 hp in a NACA cowling, along with a revised fin and rudder. The designation was changed to XP-13A. The lower-power engine actually resulted in a speed increase of 15 mph, at least partly because of the weight savings.
The airplane was delivered to the United States Army and tested at Wright Field, Dayton, Ohio, in June 1929 as P-559. The Army decided against production but the aircraft was purchased by the USAAC. The XP-13 Viper was the last fighter built by the Thomas-Morse Aircraft Corporation of Ithaca, New York. Thomas-Morse was acquired by Consolidated Aircraft, and the prototype was lost to an inflight fire.