The Sikorsky S-56 came into being as an assault transport for the U.S. Marine Corps, although some 60 per cent of those eventually built were to meet U.S. Army orders. The original requirement was for an assault transport helicopter capable of air-lifting 26 troops and their equipment. The S-56 was Sikorsky's first twin-engined helicopter, although the traditional single main rotor layout was retained, this being a 5-blade unit designed to be able to sustain the aircraft in flight with one blade shot away. For several years the S-56 was the western world's largest and fastest military helicopter, and held two height-with-payload records from 1956-59. It was also the first production helicopter to have a retractable main undercarriage, this being housed at the extremities of the small stub wings in the pods containing the engines. Loading of the aircraft was via clamshell nose doors, giving access beneath the flight deck to the 53.80m³ cabin in a similar manner to the fixed-wing Bristol 170 Freighter. A winch capable of hoisting 907kg at a time was fitted in the cabin roof to assist the loading of cargo.

The U.S. Navy placed an order in May 1951 for a prototype XHR2S-1, which was flown for the first time on 18 December 1953. The first of sixty HR2S-1's was flown on 25 October 1955, deliveries to Marine Corps Squadron HMX-1 starting in July 1956. A small batch of these aircraft were modified as HR2S-1W patrol aircraft with a huge AN/APS-20E search radar under the nose and additional crew members for radar picket duties. In 1954 an HR2S-1, redesignated YH-37, was evaluated by the U.S. Army, from which followed orders for ninety-four similar aircraft as H-37A Mojave for general transport duties.

Production of the S-56 ended in May 1960, but Sikorsky were engaged until the end of 1962 in converting all but four of the H-37A's to H-37B (later CH-37B) standard. Improvements in this version included the installation of Lear auto-stabilisation equipment and the ability to load and unload while the helicopter was hovering. The Navy and Marine S-56's became CH-37C's under the 1962 designation system. Some later production S-56's had 2100hp R-2800-54 engines.

The S-56's rotor and transmission systems were utilised in the development of the abortive Westland Westminster and Sikorsky's own S-60 and S-64 crane helicopters, but hopes of selling the S-56 on the commercial market were not realised, due mainly to the high operating costs of a piston-engined machine of this size, and a proposal to fit Lycoming T55 gas turbines was not adopted.

K.Munson "Helicopters And Other Rotorcraft Since 1907", 1968

Immediately after the S-55 had entered production, Sikorsky began working on the design of a larger helicopter, intended as an assault transport for the Marines. A twin-engine solution was chosen, and to save cabin space, it was decided to house the two large radial engines in outboard nacelles, from which two drive shafts linked up directly with the reduction gear assembly which drove the big five-blade metal rotor. The large cargo bay had a hoist capable of lifting a one tonne load. The main landing gear wheels retracted, but the tailwheel was fixed.

Some of the 60 aircraft ordered by the Marine Corps were converted into radar patrol craft (military designation HR2S-1W), with a bulbous dielectric radome under the nose, but this transformation was unsuccessful. The Army ordered 91 aircraft, designated H-37A "Mojave".

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

Sikorsky originally developed the Model S-56 twin-engined heavy lift helicopter in response to a 1950 Marine Corps requirement for an assault transport able to carry twenty-three fully equipped troops. In 1951 the Navy ordered four XHR2S-1 prototypes for USMC evaluation, and the first of these made its maiden flight in December 1953. In 1954 the Army borrowed one of these preproduction machines, designated it the YH-37, and subjected it to rigorous operational and maintenance evaluations before returning it to the Marines. On the basis of the large helicopter's excellent showing during the Army evaluation, Sikorsky was in late 1954 awarded a contract for nine production H-37A Mojaves. The first of these reached Fort Rucker during the summer of 1956, at about the same time the HR2S-1 naval variant was entering regular Marine squadron service. The Army subsequently placed orders for a further 85 H-37As, and all ninety-four aircraft were delivered by June of 1960.

At the time of its introduction into the Army inventory the H-37A was the largest helicopter in U.S. military service. It was also Sikorsky's first multi-engined helicopter, and in developing it the company chose to break with then-current industry practice by using a single five-bladed main rotor instead of two fore- and aft-mounted tandem rotors. The Mojave's designers chose not to locate the aircraft's engines in the upper section of the fuselage, as was common with most other contemporary heavy lift helicopters, but instead placed the 1900hp Pratt & Whitney radials in nacelles fixed to the ends of short shoulder-mounted stub wings; the engine nacelles also accommodated the machine's fully retractable, twin-wheeled main landing gear legs. The H-37's innovative engine arrangement gave the craft an unobstructed cargo bay of nearly 1500 cubic feet, large enough to carry three Jeeps, twenty-four stretchers, or up to twenty-six fully-equipped troops. The Mojave's nose section was equipped with large clam-shell doors which allowed vehicles to be driven straight into the cargo area, with the cockpit placed above and slightly to the rear of the doors to ensure good visibility forward and to the sides. The H-37's tailboom was very similar in appearance to that of the H-34, in that it sloped downward toward the tailwheel and ended in a sharply upswept vertical tail unit carrying a four-bladed anti-torque rotor.

In 1961 Sikorsky began converting the Army's H-37As to -B model standard by installing automatic flight stabilization systems, crash-resistant fuel cells and modified nose doors. All but four -A model aircraft were eventually converted; in 1962 these were redesignated CH-37A, while the modified machines became CH-37B. Records indicate that the Army also evaluated one of the Navy's two radar-equipped HR2S-1W airborne early warning (AEW) aircraft. This machine (BuNo 141646) retained the AEW variant's large chin-mounted radome and AN/APS-20E search radar, and was operated in Army markings and two-tone 'Arctic' paint scheme.

The CH-37 was developed just prior to the widespread adoption of the turbine engine as a standard helicopter powerplant and, as a result, the type was forced to rely on larger, heavier and less powerful pistons. This did not prove to be an insuperable handicap, however, for the Mojave ultimately proved to be a more than capable heavy lifter when properly employed. Perhaps the best illustration of such employment occurred in Southeast Asia during the summer and fall of 1963. In June of that year four CH-37Bs were temporarily deployed to Vietnam to assist in the recovery of downed U.S. aircraft. By the following December the Mojaves had recovered an estimated $7.5 million worth of equipment, most of which was sling-lifted out of enemy-dominated areas virtually inaccessible by any other means. That the CH-37 did not see more extensive service in Vietnam is primarily the result of its replacement in the Army inventory by the turbine-powered Sikorsky CH-54 Tarhe, a machine that weighed slightly less than the CH-37 but which could carry nearly four times as many troops or five times as much cargo. The last CH-37 was withdrawn from Army service in the late 1960s.

S.Harding "U.S.Army Aircraft since 1947", 1990

Developed to meet a US Marine Corps requirement for a large assault helicopter to carry 26 troops or military vehicles, for which clamshell nose-opening doors were provided, the Sikorsky S-56 was the first Sikorsky twin-engined helicopter. Two 1417kW Pratt & Whitney R-2800-50 Double Wasp engines (1566kW R-280054s on late production aircraft) were mounted on stub wings, and the nacelles also housed the main legs of the retractable landing gear, the first application of this feature in a production helicopter. The prototype XHR2S-1 flew on 18 December 1953 and 60 production machines were delivered from July 1956. Two HR2S-1W . helicopters were converted for US Navy early warning operations with AN/APS-20E radar under the nose. US Army evaluation of an HR2S-1, under the designation YH-37, resulted in orders for 94 H-37A Mojave helicopters which went into service, initially with 4th Medium Helicopter Transportation Company, in February 1958. Modernised H-37As redesignated H-37B (later CH-37B), were redelivered to the US Army from 1961.

D.Donald "The Complete Encyclopedia of World Aircraft", 1997

- Mojaves were replaced by the CH-54 Tarhe, which weighed less but could lift five times as much cargo as the CH-37.

- In all, 150 S-56s were built; a prototype, 55 for the USMC and 94 for the Army.

- 1959 saw the first overseas H-37 deployment, by the Army to Germany.

- Army H-37As entered service with the 4th Medium Helicopter Transportation Company in February 1958.

- The H-37A had a fuselage capacity large enough to hold three Army jeeps.

- The Army briefly evaluated one of the two HR2S-1Ws in 'Arctic' colours.