Douglas Aircraft developed the C-124 from 1947 to 1949, from a prototype they created from a World War II–design Douglas C-74 Globemaster, and based on lessons learned during the Berlin Airlift. The aircraft was powered by four large Pratt & Whitney R-4360 piston engines producing 3,800 hp (2,800 kW) each. The C-124's design featured two large clamshell doors and a hydraulically actuated ramp in the nose as well as a cargo elevator under the aft fuselage. The C-124 was capable of carrying 68,500 lb (31,100 kg) of cargo, and the 77 ft (23 m) cargo bay featured two overhead hoists, each capable of lifting 8,000 lb (3,600 kg). As a cargo hauler, it could carry tanks, guns, trucks and other heavy equipment, while in its passenger-carrying role it could carry 200 fully equipped troops on its double decks or 127 litter patients and their attendants. It was the only aircraft of its time capable of transporting heavy equipment such as tanks and bulldozers without disassembly.
The C-124 first flew on 27 November 1949, with the C-124A being delivered from May 1950. The C-124C was next, featuring more powerful engines, and an APS-42 weather radar fitted in a "thimble"-like structure on the nose. Wingtip-mounted combustion heaters were added to heat the cabin, and enable wing and tail surface deicing. The C-124As were later equipped with these improvements.
One C-124C, 52-1069, c/n 43978, was used as a JC-124C, for testing the 15,000 shp (11,000 kW) Pratt & Whitney XT57 (PT5) turboprop, which was installed in the nose.
- YC-124 : Prototype rebuilt from a C-74 with a new fuselage and powered by four 3,500 hp R-4360-39 engines, it was later re-engined and redesignated YC-124A.
- YC-124A : Prototype YC-124 re-engined with four 3,800 hp R-4360-35A engines.
- C-124A : Douglas Model 1129A, production version with four 3,500 hp R-4360-20WA engines; 204 built, most retrofitted later with nose-radar and combustion heaters in wingtip fairings.
- YC-124B : Douglas Model 1182E was a turboprop variant of the C-124A with four Pratt & Whitney YT34-P-6 turboprops; originally proposed as a tanker, it was used for trials on the operation of turboprop aircraft.
- C-124C : Douglas Model 1317, same as C-124A but with four 3,800 hp R-4360-63A engines, nose radar, wingtip combustion heaters and increased fuel capacity; 243 built.