The Thomas-Morse O-21 was a version of the successful Thomas Morse O-19 observation aircraft that was powered by a Curtiss Chieftain engine then by a Wright Cyclone.
In 1926 Thomas-Morse were given a contract to produce two examples of the Douglas O-2 observation aircraft, but with a metal framework for the wings, replacing the wooden structure of the O-2. Two prototypes and three production aircraft were built as the Thomas Morse O-6, but the type wasn’t accepted by the USAAC.
Undaunted Thomas-Morse decided to produce a greatly modified prototype, designed from the start around the use of an all metal framework. The resulting XO-6B was powered by a Pratt & Whitney Wasp R-1340 radial (replacing the inline Liberty engine of the O-2 and O-6), had fabric covered wings and a corrugated metal fuselage, and on 16 June 1928 the USAAC ordered four, with the new designation O-19.
On 20 December 1928, while these four aircraft were still under construction, the USAAC placed a second order, for one aircraft powered by the Pratt & Whitney Hornet and one powered by the Curtiss Chieftain. This order was probably completed using two of the original four aircraft, as the first two O-19s were given the serial numbers 28-400 and 28-401, the O-20 was 28-402 and the O-21 was 28-403. The last two aircraft from the original order were later completed as 29-369 and 29-370.
The Chieftain engine that was originally installed on the O-21 was an unusual design. It was a twelve cylinder two row air cooled radial engine, but with the two rows of cylinders lined up with each other. Normal two row radial engines of the period had the second row of cylinders displaced so there would be in the gaps left in the first row, so that both would get some direct air flow (even the massive four row Wasp Major had the four rows staggered). Curtiss hoped that the use of a Townend ring would make the Chieftain more aerodynamic than its rivals. The Chieftain was the first ‘inline radial’ to actually be airworthy, but Curtiss were never able to solve the inevitable overheating problems with the rear row of cylinders, and the project was cancelled after very few eninges had been completed.
The O-21 did have a rather neat appearance while powered by the Chieftain. On the original O-19 the cylinder heads were exposed to improve cooling, while on the O-19C a cowling ring was added. On the engine was entirely contained within a neat circular nose, with various openings for exhausts and cooling.
After the failure of the Chieftain the O-21 was turned into the XO-21A by giving it a 525 Wright Cyclone R-1750-1. This was a nine cylinder single row air cooled radial engine, and would later be developed into the R-1820 Cyclone, which powered the B-17 Flying Fortress.