Grumman had been working on a successor to the F4F Wildcat since 1938 and the contract for the prototype XF6F-1 was signed on 30 June 1941. The aircraft was originally designed to use the Wright R-2600 Twin Cyclone two-row, 14-cylinder radial engine of (the same engine used with Grumman's then-new torpedo bomber under development), driving a three-bladed Curtiss Electric propeller. Instead of the Wildcat's narrow-track, hand-cranked main landing gear retracting into the fuselage that it had inherited, little changed in design from the 1931-debuted Grumman FF-1 fighter biplane, the Hellcat had wide-set, hydraulically actuated landing gear struts that rotated through 90° while retracting backwards into the wings, but with full wheel doors fitted to the struts that covered the entire strut and the upper half of the main wheel when retracted, and twisted with the main gear struts through 90º during retraction. The wing was mounted lower on the fuselage and was able to be hydraulically or manually folded, with each panel outboard of the undercarriage bay folding backwards from pivoting on a specially oriented, Grumman-patented "Sto-Wing" diagonal axis pivoting system much like the earlier F4F, with a folded stowage position parallel to the fuselage with the leading edges pointing diagonally down.