Additionally, Marshall Wainwright notes that other sources indicate the first eight aircraft were to originally have been powered by Allison V-1710-13 engines fitted with GE Type B-6 turbo-superchargers (turbochargers). These aircraft were eventually delivered with improved V-1710-23(D2) engines. Wainwright further states that two of the YFM-1 airframes were changed on the production line to accept the V-1710-41 without turbo-supercharging, becoming YFM-1Bs. This is noted in a contract change dated 19 October 1939 which shows that aircraft 38-489 and 38-490 had their turbos, all associated ducting, and controls removed and V-1710-41(D2A) "Altitude Rated" engines installed instead. The (D2A) was essentially a -23 with higher supercharger gear ratios (8.77:1 versus 6.23:1), which allowed the motor to develop around up to ASL. They used the same ratings and components as the Altitude Rated V-1710-33(C15) Allison fitted to the original Curtiss XP-40. Allison was paid $1,690 to modify each engine.