Cassagneres goes on to state, "The wings were constructed of solid spruce spars, stamped aluminum alloy ribs, and steel compression members, and were fabric covered.

The STM-2 was derived from the STM with changes including wider cockpits to enable military pilots to enter and exit while wearing parachutes, external stringers, and provision for a machine gun on some examples.

[5] Variants in the series included the STM-2P single-seat version armed with a machine gun delivered to Nationalist China; and the STM-S2, which could be fitted with landing gear or with EDO Model 1965 floats.

The United States Army Air Corps (USAAC) had purchased several dozen ST-M variants under various designations and had Ryan Aeronautical re-engine most with Kinner R-440 radial engines.

Other changes included a revised rudder, balanced ailerons and elevators, and strengthened main landing gear with the legs spaced further apart.

The ST-3KR had a more powerful Kinner R-5 engine fitted and became the definitive model; more than 1,000 military versions were built during World War II as PT-22 Recruits.

The main wing was clipped back to the landing gear; a small nose wheel added to prevent ground loops; a roll cage between cockpits to protect the pilot and cadet; and the throttle modified so the engine could not go over a certain RPM.

[9] Total production of civil and military aircraft prior to the entry of the United States into World War II amounted to 315.

[13] Most civil aircraft in the ST series were delivered in the United States, although a few were exported to South Africa, Australia and various countries in Latin America.

These were the first of more than 1,000 Ryan STs to serve the USAAC, its successor, the United States Army Air Forces (USAAF) and the USN.

[14][15] A large number of STMs were exported in the 1930s and early 1940s (prior to the entry of the United States into World War II) to various air forces, with the biggest customer being the military of the Netherlands East Indies, now Indonesia.

[6] The STM was chosen by the South American Air-forces because of the superior performance of the super-charged Menasco engine at the high altitude airports encountered.

After the Japanese invasion of the Netherlands East Indies many Ryans in that country were pressed into combat, especially in reconnaissance roles, and large numbers were shot down or destroyed on the ground.