The X-51A  Waverider is expected to fly at six times the speed of sound

The US air force is testing an experimental aircraft  designed to fly at six times the speed of sound, or about 4,600mph.

The X-51A Waverider will be dropped from a B-52 bomber at around 50,000ft over the Pacific Ocean before attempting to achieve speeds that would see it fly from New York to Los Angeles in 46 minutes.
It is the third test of the hypersonic aircraft in the last two years. The earlier incarnations failed to fly for the 300 seconds that engineers hope the X-51A will be able to sustain, leading to tweaks to the craft's engine ahead of the next launch.

The Los Angeles Times reported that the X-51A, which is unmanned, is expected to reach Mach 6 when it takes flight off the Southern California coast near Point Mugu. The aim of flying for five minutes is longer than the aircraft's personal best, which came in 2010 when it flew for more than three minutes.

http://www.guardian.co.uk/world/2012/aug/14/us-air-force-test-experimental-aircraft