The Thomas-Morse O-33 was a version of the successful O-19 observation aircraft that was powered by a Curtiss Conqueror engine and given a revised tail.
Early in 1926 Thomas-Morse had been given a contract to produce two examples of the Douglas O-2 observation aircraft, but with a metal structure for the wing in place of the wooden one of the O-2. These aircraft, the O-6, were powered by the inline Liberty engine, and were completed during 1926, but no orders followed. Thomas-Morse decided to produce their own design, the XO-6B, which was purpose built to benefit from the all metal framework. This was powered by the Pratt & Whitney Wasp R-1340 radial engine. This time the USAAC was impressed, and placed an order for four examples of the new aircraft on 16 June 1928, as the O-19.
The first version of the O-19 to use the Conqueror engine was the single O-23 of 1928-29. The engine installation of the O-19 was rather ungainly, with an angular top to the nose and a large air intake jutting down below the chin of the aircraft, ending just in front of the main struts for the undercarriage.
A second Conqueror powered version was produced in August 1930 by installed a Conqueror V-1570-11 engine in the first of the production O-19Bs (serial number 30-90), which then became the O-33.
The O-33 had a rather more refined engine installation than the O-23, with a more curved top to the nose of the fuselage, and a smoother fairing around the air intake below the engine. On the O-23 this had ended very abruptly, while on the O-33 it smoothly merged into the existing fuselage, giving the fuselage more depth behind the engine.
The O-33 also had a modified tail, although the exact changes are unclear, as no good photos of the O-33’s tail have yet been found. The O-41, which is said to have been developed from the O-33, had a more rounded vertical tail than the O-19, but the one photo of the O-33 I have found appears to show the original more angular version.
On 9 May 1934 aircraft 30-90 crashed two miles east of Flint, Michigan, while serving with the 57th Service Squadron (the pilot bailed out). The aircraft was described as the O-33B in the accident report. This makes it impossible for the O-33 to actually be the basis of the O-41, as is often stated.