Intrigued by Republic's design, the US Air Force placed an initial order for 199 F-105s in September 1952, but with the Korean War winding down reduced it to 46 six months later. On October 22, 1955, the first YF-105A prototype flew, powered by a Pratt & Whitney J57-P-25 engine. Test flights with the YF-105A soon revealed that the aircraft was underpowered and suffered from problems with transonic drag. To counter these issues Republic replaced the engine with the more powerful Pratt & Whitney J75, altered the arrangement of the air intakes, and redesigned the F-105's fuselage.