In 1925 the U.S. Army adopted a new designation system for cargo/transport aircraft. Between 1919 and 1924, aircraft used a T- (Transport) designation and in 1925 the system was changed to C- (Cargo). The C- system is still in use today although the numbering started over in 1962 when the Army, Navy and Air Force systems were combined and simplified.
The Douglas C-1 was the first plane 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 pounds 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.
The C-1 was powered by the Liberty 12 engine and carried a crew of two in an open cockpit. Several C-1s were used in test programs -- as an engine test bed, as a prototype air ambulance and as refueling aircraft for early air-to-air refueling experiments.
Seventeen additional aircraft were ordered in 1926 and 1927 as C-1Cs and were slightly larger than the original C-1s.
First plane using C designation
TECHNICAL NOTES: Engine: Liberty V-1650-1 of 435 hp Maximum speed: 116 mph Cruising speed: 85 mph Range: 385 miles
Service ceiling: 14,850 ft. Span: 56 ft. 7 in. Length: 35 ft. 4 in. Height: 14 ft. 0 in. Weight: 6,445 lbs. loaded Cargo/passenger capacity: Six passengers or approx. 2,500 lbs. of cargo Crew: Two (pilot and flight mechanic) Serial numbers: 25-425 to 25-433 (S/N 25-426 was used for engine testing and designated C-1A; S/N 25-430 was used as an air ambulance test aircraft; S/N 25-428 and 25-432 were used to refuel the C-2A Question Mark during the historic 1929 flight)