Kellett Autogiro Corp. formed Philadelphia 1929 to operate Cierva licence obtained from Pitcairn. K-2, -3 and -4 were followed 1934 by KD-1 (Kellett Direct-control) built in small series including YO-60 for Army. Switched to helicopters with 'eggbeater' intermeshing rotors, XR-8 (1943) leading to XR-10 able to carry 6 stretchers (litters) in 1948.