The aircraft developed for cargo and transport duties in the early 1930s were primarily small single engine land planes and twin engine amphibians. Most of these planes were standard commercial types acquired for use by the Air Corps without significant modifications. The trend of buying standard commercial types continued into the mid-1930s; however, aircraft development was concentrated on larger twin engine transports. The Douglas DC-2 and Lockheed Electra were the two dominate types bought and constituted eight of ten aircraft in the 30-series of cargo transports.
The Curtiss C-30 was one of the first larger capacity, twin engine transports acquired by the Air Corps. Its biplane configuration limited its speed and only two aircraft were bought because the Army preferred to wait for more advanced types being designed in 1933. The civilian version -- T-32 Condor II -- was relatively successful as an airliner in the time in took the newer designs to be built and tested.
The Kreider-Reisner XC-31, like the XC-30, was somewhat out of date before it was built. The engineers at Kreider-Reisner (a subdivision of the Fairchild Aircraft Company) designed the plane in response to an Air Corps request for a cargo/troop transport capable of carrying about 3,500 pounds of cargo or 12-plus combat troops. The single engine, fabric covered plane lost a fly-off against the Douglas C-32 (DC-2).
XC-32 was a designation assigned to a standard Douglas DC-2 acquired by the Air Corps for evaluation and flight testing in a fly-off competition. The plane was the first in a long line of variants based on the Douglas DC-2 and DC-3. The C-33 was a DC-2 design modified for use by the Air Corps. It featured a cargo loading door and a larger tail. The C-34 was also based on the DC-2 but had a special interior designed as a VIP.
The Lockheed XC-35 was a specifically designed as a flight test aircraft for high altitudes. In continuing its program for investigating flight at high altitudes, the Air Corps ordered from Lockheed a twin-engine airplane based on the Electra design and designated it as XC-35. This was the world's first airplane specifically constructed with a pressure cabin. For its achievements with the XC-35 in high-altitude research, the Air Corps was awarded the Collier Trophy for 1937.
The C-36 was a military light transport version of the Lockheed Electra 10-A civilian type. The Air Corps bought a small number of C-36s in 1937, but got more during 1942 when almost all civilian aircraft usable by the military were impressed because of the wartime emergency. The single C-37 was also a Lockheed Electra but had a VIP interior for use by the Chief of the National Guard.
The Douglas C-38 was a C-33 modified with a DC-3 tail assembly. This plane was nicknamed DC-2½. The C-39 was essentially the production version of the C-38 and was a composite of Douglas military and civilian aircraft designs. The fuselage was copied from the DC-2 commercial airliner (Air Corps designation, C-33), the tail was almost identical to that of the DC-3 airliner, and the landing gear was based on that of the Douglas B-18 bomber.