In 1958 Sikorsky began design work on the Model S-60 twin-engined heavy-lift helicopter, a machine that incorporated the pod-mounted piston engines and dynamic components of the earlier Model S-56/CH-37. The S-60's fuselage was extremely simple, 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. One S-60 was built for Navy evaluation, but the craft was found to be underpowered for its intended roles and Sikorsky took the design back to the drawing boards for extensive reworking. The reconfigured machine, which was allotted the company designation S-64A, made its first flight in May 1962 under the watchful eyes of Army observers.
S.Harding "U.S.Army Aircraft since 1947", 1990
Sikorsky's first 'flying crane' helicopter was the Sikorsky S-60, developed from the S-56 and retaining that machine's powerplant, transmission and rotor system. Work began in May 1958 and the prototype was flown on 2 March 1969; 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. 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.
D.Donald "The Complete Encyclopedia of World Aircraft", 1997