More on the St. Louis trainer and the decidedly unfortunate company that produced it:
During World War I, the St. Louis Car Co. from St. Louis, Missouri, proposed to manufacture as many as 5,000 modified Benoist Type XV boats for the British, for use as antisubmarine patrol aircraft. However the British were already committed to the earlier, more thoroughly tested Curtiss flying boats, and the deal fell through.
A merger was attempted with Benoist in 1915 but was eventually unresolved.
The St. Louis Aircraft Division of the St Louis Car Company was founded in 1917 with the task of producing JN-4 trainers for the U.S. armed forces as a subcontractor to Curtiss.
After the war, the division remained dormant. It probably tried to obtain contracts, but apparently only produced the suspended engine-equipped crew car of the Army's C-6 motorized observation balloon.
With the late 1920s boom on commercial aircraft, it was reorganized as the St. Louis Aircraft Co. and produced the C2 Cardinal series of high-cabin monoplanes, a little over 20 of which were produced in 1929-1930 in seven different variants. It was a neat type that could have met with a lot more success, but the economic crisis fell hard on many aviation companies.
After a lapse of five years, the single XPT-35 (or PT-35, depending on sources) was produced in 1935 (hence its designation) with the hope of winning an Army contract for a biplane primary trainer to be produced in quantity (eventually this was achieved by the Stearman/Boeing Kaydet series).
Only one aircraft was built, receiving registration [11778] (c/n 35-101). Powered by a 225hp Wright R-760 engine, it crashed at Wright Field in 1936 during testing. The design, however, was promising enough to justify the order of a replacement prototype. Unfortunately, this machine, the PT-1W, temporarily registered [NX18068] and assigned Army serial 39-703 (c/n 101) came a little too late. By the time the aircraft, now designated as XPT-15, was flown in 1940, the Army Air Corps had been enthused over the Boeing 73 demonstrator (evaluated at Wright Field as the XPT-945), and orders of the type were already rolling.
The PT-1W design was sound, the prototype performed well, and so the Army ordered a batch of 13 pre-production aircraft under the designation YPT-15, with serials 40-1 to 40-13 (presumably c/n 1 to 13), delivered in 1941. Unfortunately for the St. Louis company, full production status was never achieved because the Stearmans were wholly satisfactory.
St. Louis didn't give up on its high hopes of becoming a major Army contractor, and came up in 1940 with a new trainer, this time a low-wing tandem-cockpit monoplane designated the PT-LM-4 [NX25500] (c/n 14), powered by a 180hp Ranger 6 engine. Although this was meant for military evaluation, once again it didn't attract any orders, this time because of another serious competitor, the Fairchild M-62, which won the contract and was produced as the PT-19/PT-23 Cornell series. Ironically, St. Louis DID receive a fairly large contract for Army trainers, but as a subcontractor on the very aircraft that had beaten their design, the Ranger-powered PT-19A and the Continental-powered PT-23 and -23A, producing a total of 350 of all three versions.
The company lost all hopes of ever making it in the aircraft business after the drastic failure of their cargo glider designs, the eight-seat, gull-wing XCG-5 and fifteen-seat XCG-6. Only an static test model was built of the former [41-29619]. All ground testing demonstrated the failure of the design, resulting in immediate cancellation of the larger model.