EDITED-The experimental Boeing X-51 Waverider aircraft is being readied to make a Mach 6 flight (six times the speed of sound) over the Pacific ocean, according to the Department of Defense. A cross between an airplane and a rocket, the Waverider is so-named because it stays partly aloft from the lift created by its own shock wave.
An earlier experimental X aicraft flew at a faster Mach speed in 2004, but only for a few seconds and that engine wasn't designed to survive the flight.
The X-51 Waverider's test flight will be of five minutes duration and may reach Mach 6. (Some time to accelerate to speed, you know). The X-51's technology eventually could be used in space vehicles. X planes are experimental or research aircraft designed to explore boundaries of aeronautical science or space.
The North American X-15 has been around since 1959, when first flights were made. Pete Knight reached Mach 6.72 (4,534 mph) in the X-15's most spectacular flight in 1967, so Mach 6 flight has been achieved before.