With the third X-51A WaveRider failing to reach hypersonic speed due to a fin failure last August, it seemed the United States Air Force would possibly forgo the fourth (and final) run. On the morning of May 1st, however, that last X-51A got its chance to soar, successfully reaching Mach 5.1 during a record 370-second flight. According to the Wright Patterson Air Force base, the aircraft's rocket booster helped it hit Mach 4.8 about 26 seconds after being released from a B-2H at 50K feet, at which point its air-fed scramjet brought it to 60,000 feet while achieving hypersonic flight. The USAF notes that "it was the longest of the four X-51A test flights [230 nautical miles] and the longest air-breathing hypersonic flight" -- surely taking some of the sting out of the $300 million program's previous shortcomings. Past flights aimed to hit Mach six, with the first and second tests only sustaining Mach five.
The aircraft made destructive splashdown landing into the Pacific as planned, but data from the whole flight was recorded. The USAF isn't planning a follow-up to the X-51A anytime soon, though the program will likely serve as a reference for future designs. You can dig into the official rundown at the link below.