The YF-100A first flew on 25 May 1953, seven months ahead of schedule. It reached Mach 1.04 in spite of being fitted with a de-rated XJ57-P-7 engine. The second prototype flew on 14 October 1953, followed by the first production F-100A on 9 October 1953. The USAF operational evaluation from November 1953 to December 1955 found the new fighter to have superior performance, but declared it not ready for wide-scale deployment due to various deficiencies in the design. These findings were subsequently confirmed during "Project Hot Rod" operational suitability tests.