The Eagle's initial versions were the F-15 single-seat variant and TF-15 twin-seat variant. (After the F-15C was first flown, the designations were changed to "F-15A" and "F-15B"). These versions would be powered by new Pratt & Whitney F100 engines to achieve a combat thrust-to-weight ratio in excess of 1:1. A proposed 25-mm Ford-Philco GAU-7 cannon with caseless ammunition suffered development problems. It was dropped in favor of the standard M61 Vulcan gun. The F-15 used conformal carriage of four Sparrow missiles like the Phantom. The fixed wing was put onto a flat, wide fuselage that also provided an effective lifting surface. The first F-15A flight was made on 27 July 1972, with the first flight of the two-seat F-15B following in July 1973.