The Pratt & Whitney J57 is an American jet engine, the first to surpass 10,000 pounds of force (4,500kgf), and the first to use a two-spool axial compressor design, which improved its performance and efficiency at different altitudes and speeds. It was developed by Pratt & Whitney in the early 1950s, based on the earlier XT45 turboprop engine that was intended for the Boeing B-52 bomber. The J57 powered a variety of military and commercial aircraft, such as the F-100 Super Sabre, the U-2 spy plane, the B-52 Stratofortress, the Boeing 707, and the Douglas DC-8. It was also later modified to include a low-bypass fan section, which increased its thrust and reduced its fuel consumption. This version was known as the JT3D or TF33, depending on the application. Production is set to cease in the mid-1960s.