What is the fastest jet airplane?
Officially, the fastest jet plane ever built is the SR-71 Blackbird, built by Lockheed. It was capable of speeds in excess of 3.5 mach (more than 2,200 miles per hour). It was decommissioned in 1998. No one knows for sure whether a faster plane exists, but many people suspect that the SR-91 -- if it actually exists -- is capable of speeds in 5-mach range.
The fastest known jet airplane is the SR-71 Blackbird. However faster aircraft have been flown by the U.S. military. But they are rocket powered and experimental which does not fall into your question of the fastest "jet" airplane. Currently the fastest commercial airliner is the Lockheed L-1011 with a listed maximum speed of .95 Mach. However, until it was retired the Concorde was the fastest (supersonic) commercial airliner.
Fastest production business jet class aircraft is the Cessna Citation X (model 750). Its maximum cruise speed is .92 Mach (although it's suspected that the aircraft has flown faster) ...Steve Fossett holds the US continental crossing record in a non-military aircraft with a Citation X in 2003, at an average speed of 726.83 mph.
The standing manned jet speed record is currently held by the SR-71, as posted. Experimental manned projects may have exceeded that, but have not been announced.
The scram jet is even faster. But the current crop are all small and unmanned. Scram jet engines don't work at less than the speed of sound. They have to be launched from supersonic aircraft or attached to the nose of a rocket booster to get the scram jet up to minimum operating speed. The Boeing X-43, which is NASA's unmanned scram jet project, recently hit 7,546 mph or mach 9.8 in a test run. There is a report that another scram jet neared mach 10, but did so with a gravity assisted dive.
(this is someone different) It is NASA's X-43B. I know because my dad helped make it.
(this is someone different again) Although the X-43B is the fastest airplanes there are other Supersonic and hypersonic airplanes including the XB-70 Valkyrie, Tu-360, and X-15, MiG-25R Foxbat-B, there may be many others but lots are kept unrevealed.
There was also a Rocket Plane Created by NASA that could reach mach 6 (6x the speed of sound)!
Estimates of the Blackbird's top speed vary, but its true
maximum has never been made public, and may remain in the dark for
decades. Credible estimates range from Mach 3.2 to Mach 3.5; I
suspect the truth holds an even larger number.
NASA has built a prototype jet that will hit Mach 9.6 - that makes the Lockheed fighter look like a little old lady.