What the fastest American fighter jet ever built?

The actual answer would be the 1960's era YF- 12 A, the SR-71's sister. The YF-12 A was intended to intercept entire of flights of Soviet bombers and shoot them down with the Hughes AIM-47 Air-to-air NUCLEAR missile! The YF-12A with a top speed in excess of 2435.8 mph/mach3.2, is still the fastest fighter-interceptor EVER built and flown.