In 1950, Boeing applied flying boom technique, developed 1948-9, to C-97A. In Dec, KC-97A conversion announced. 3 A models modified at Renton as prototypes for KC-97E of which 60 built 1951-2. E and later models convertible tanker/transports. Without refuelling equipment, full transport capability retained; partial transport capability still retained with equipment installed. Boom operator's position and base for the boom itself attached to underside of the fuselage in place of removable clam-shell loading doors, permitting easy conversion. F model with R-4360-59B engines (3,800 hp each) replaced -35C engines (3,500 hp) on E model; 159 built at Renton. Further development produced G model; 592 built. In G, tanks used for refuelling relocated so aircraft could operate as tanker or transport without conversion. Mods also made to boom, but engines same as for E model. Conversions to L configuration began 1964, and most went to Air National Guard.