The Pentagon hopes its X-51A will manage to hit hypersonic velocity by next spring or summer.
The United States military tested the X-51A Waverider - its first hypersonic aircraft - back in 2010. The autonomous flight, conducted on May 25, was supposed to see the Waverider accelerate to Mach 6. However, the aircraft didn't quite manage to reach its intended velocity, as it only clocked in at Mach 5.
The most recent test of another X-51A (a total of four were built) was held in August of 2012. Unfortunately, the aircraft experienced critical control issues only seconds into the flight and uncermoniously plummeted into the ocean.
As noted above, the US military has confirmed that it plans to conduct a fourth flight for the hypersonic X-51A some time during the spring or summer of 2013.
During the test flight in August, the aircraft was able to detach from the B-52 without issue and accelerated to Mach 4.8 before it became unstable and crashed. Apparently, the X-51A failed to activate its scramjet engine, which is a key feature the military hopes to test. Subsequent investigations blamed a "random vibration issue" with the aircraft crew saying more research was needed to pinpoint the exact cause.
So far, the aircraft has failed to reach its intended speed of Mach 6, but the 2010 flight did see speeds of Mach 5 for approximately 3 minutes. According to program manager Charlie Brink, the military will continue hypersonic flight research even after next year's scheduled flight of the X-51A.