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The Thomas-Morse O-20 was a version of the successful O-19 observation aircraft that was powered by a Pratt & Whitney Hornet engine.
Early in 1926 Thomas-Morse was given a contract to produce a version of the Douglas O-2 biplane observation aircraft, but with an all metal framework (the O-2 had a welded steel tube frame for the fuselage, but wooden wings). Thomas-Morse produced two prototypes and three production aircraft as the O-6, but it wasn’t adapted by the USAAC.
Thomas-Morse then produced a privately funded prototype of an observation aircraft designed from the start around an all metal fuselage, as the XO-6B. This was powered by a Pratt & Whitney Wasp R-1340 radial engine, with an all metal structure, fabric covered wings and a corrugated metal fuselage cover. This time the USAAC was more impressed, and on 16 June 1928 four examples of the modified aircraft were ordered as the XO-19.
On 20 December 1928, before any of these aircraft had been completed, the USAAC placed a second order, for one aircraft powered by the Pratt & Whitney Hornet and one powered by the Curtiss Chieftain. The serial numbers these aircraft used suggests that two of the original four aircraft were used to fill this new order, and two new aircraft built to finish the original order – the first two O-19s had the serial numbers 28-400 and 28-401, the second two 29-369 and 29-370, while the December 1928 aircraft were 28-402 and 28-403.
The Hornet powered aircraft became the Thomas-Morse O-20 (serial number 28-402, McCook test number P-561). It was powered by a 525hp Pratt & Whitney Hornet R-1690-1 engine, a single row air cooled radial engine with a diameter of 41.41in and dry weight of 1,014lb. The Hornet had a smaller diameter than the Wasp (51.75in) but was heavier. It was a fairly successful engine, although only 2,955 were produced between 1926-1942, as it was never used in the mass produced wartime aircraft.
Only one O-20 was produced, and no further Hornet powered versions of the O-19 were built.