Douglas received an order for nine single-engined transport aircraft in 1925, the first aircraft flying from Douglas's Santa Monica, California factory on 2 May 1925. The C-1 was the first aircraft assigned in the new C- category. The aircraft design was based on several earlier and similar designs developed by Douglas in the early 1920s (including the Douglas World Cruisers used in the first round-the-world flight in 1924). The C-1 featured an enclosed passenger compartment capable of carrying six passengers or about 2,500 lb (1,100 kg) of cargo. A trap door was placed in the lower fuselage to allow large and/or heavy cargo (particularly aircraft engines) to be lifted directly into the cargo compartment. An auxiliary door for passengers and light cargo was included on the right side of the center fuselage.
Seventeen additional aircraft were ordered in 1926 and 1927 for the United States Army Air Corps as C-1Cs and were slightly larger than the original C-1s.
Several C-1s were used in test programs—as an engine testbed, as a prototype air ambulance and as refueling aircraft for early air-to-air refueling experiments. Two of these aircraft were used as "tankers" in the 1929 record endurance flight of the Fokker C-2Question Mark.
A Douglas C-1 (top) refueling the Question Mark (bottom).
Single-engined cargo/passenger transport aircraft, powered by a 435 hp (324 kW) Liberty V-1650-1 piston engine, accommodation for two crew and six to eight passengers.
One C-1 used to test a variety of engines and cowlings, it was also used for number of experiments with ski landing gear.
Single-engined cargo/passenger transport aircraft, fitted with a metal cabin floor, modified landing gear, it had a higher gross landing weight and increased dimensions, plus a new balanced rudder; 17 built