Douglas sourced significant portions of the aircraft, including responsibility for the turbojet engines, radar, tail-mounted turret, electrical generators and various other secondary systems, from the Westinghouse Electric Corporation; this heavy reliance on Westinghouse allegedly caused some discomfort to Douglas. In particular, development of the Westinghouse J40 engine proved to be incapable of meeting development schedules or performance promises; Heinemann later commented that "the first thing we learned was that the J40 was not powerful enough". Even prior to the first flight being conducted, Douglas was considering switching to rival manufacturer Pratt & Whitney's J57 engine, which was heavier, but allowed the overall aircraft to be lighter due to being less fuel hungry.