The Sikorsky CH-37 Mojave was the largest Western helicopter of its time but had numerous deficiencies that prevented it from being an ideal design for a heavy transport. It's main problem was the large and cumbersome twin piston engines (the first such Sikorsky design) which were housed on stub wings on the upper fuselage. The engine pods also carried the long, retractable main landing gear. The piston engines limited performance, and were quickly shown to be obsolete following the introduction of turboshaft engines on most subsequent helicopter designs. The benefit of this arrangement was that it left the fuselage free for cargo, which could comprise as much as 26 troops, 24 stretchers, three Jeeps or Mighty Mite trucks, a 105-mm howitzer. These would be loaded through clamshell doors in the nose. Heavier cargo could also be carried on a sling, including Honest John rockets or even light armored vehicles. In naval use, the Mojave was modified as an AEW platform with a massive radar fitted in the chin. The type saw limited service in Vietnam but was retired in the late 1960s and replaced by the more capable CH-53 and CH-54.

First flight of the XHR2S (company designation S-56) took place on 18 December 1953, entering service in early 1956 as the HR2S-1 for the USMC (an AEW version was the HR2S-1W). A transport version for the US Army was the H-37A with some structural modifications. This was followed by the H-37B with autostabilization and crashproof tanks. These were redesignated CH-37A and CH-37B in 1962, and naval units as the CH-37C.