In May 1958 Sikorsky began design work on the Model S-60 twin-engined heavy-lift helicopter. The S-60 grew from the S-56/CH-37, using the same piston engines and dynamic components. It was built as a research vehicle to demonstrate the flying crane concept with a thin, strong fuselage, consisting of a central 'backbone' which supported the podded engines, main and tail rotor systems, and a nose-mounted crew cabin.
Bulk cargo and passengers were intended to be carried in large rectangular pods that could be attached to the underside of the aircraft's central spine, whereas vehicles and other out-sized loads were to be sling-hoisted. It was capable of lifting a 5443kg payload beneath the fuselage boom, and the co-pilot could turn his seat to face aft to control loading and unloading.
One S-60 was built for Navy evaluation, first flown on 25 March 1959, the craft was found to be underpowered for its intended roles. The prototype S-60 crashed in April 1961, but by then Sikorsky had begun construction of an enlarged version, with a six-bladed main rotor driven by two 3020kW JFTD-12A turboshaft engines. Designated S-64, the prototype flew on 9 May 1962.