The S-56 came into being as an assault transport for the United States Marine Corps (USMC), with a capacity of 26 fully equipped marines. An order for the aircraft was placed in 1951 using the U.S. Navy/U.S. Marine Corps designation of the time of HR2S. The first prototype, the XHR2S-1 flew in 1953 and production deliveries of the HR2S-1 began in July 1956 to Marine Helicopter Squadron One (HMX-1), with a total of sixty aircraft being produced.

The United States Army evaluated the prototype in 1954 and ordered 94 examples as the CH-37A, the first being delivered in summer 1956. All Marine Corps and Army examples were delivered by mid-1960. Army examples were all upgraded to CH-37B status in the early 1960s, being given Lear auto-stabilization equipment and the ability to load and unload while hovering. In the 1962 unification of United States military aircraft designations, the USMC examples were redesignated from HR2S-1 to CH-37C.

Below is a partial list of airframes that survived military service.