SpaceShipOne is a suborbital air-launched spaceplane that completed the first manned private spaceflight in 2004. That same year, it won the US$10 million Ansari X Prize and was immediately retired from active service. Its mother ship was named "White Knight". Both craft were developed and flown by Mojave Aerospace Ventures, which was a joint venture between Paul Allen and Scaled Composites, Burt Rutan's aviation company. Allen provided the funding of approximately US$25 million.

Rutan has indicated that ideas about the project began as early as 1994 and the full-time development cycle time to the 2004 accomplishments was about three years. The vehicle first achieved supersonic flight on December 17, 2003, which was also the one-hundredth anniversary of the Wright Brothers' historic first powered flight. SpaceShipOne's first official spaceflight, known as flight 15P, was piloted by Mike Melvill. A few days before that flight, the Mojave Air and Space Port was the first commercial spaceport licensed in the United States. A few hours after that flight, Melvill became the first licensed U.S. commercial astronaut. The overall project name was "Tier One" which has evolved into Tier 1b with a goal of taking a successor ship's first passengers into space within the next few years.

Flight 16P of SpaceShipOne was a spaceflight in the Tier One program that took place on September 29, 2004. It was the first competitive flight in the Ansari X PRIZE competition to demonstrate a non-governmental reusable manned spacecraft, and is hence also referred to as the X1 flight. A serious roll excursion occurred during boost, so the flight did not achieve the expected altitude. However, it exceeded 100 km altitude, making it a successful X PRIZE flight.

After takeoff, White Knight and SpaceShipOne ascended to the launch altitude, planned to be around 14 km. At 08:09 SpaceShipOne was released, glided for 6 s, then went into nose-up attitude and the rocket motor was ignited. The rocket motor was capable of burning for approximately 87 s, having been upgraded since the previous flight. It was planned to shut off the motor at an altitude of 345,000 feet (105 km), presumably to avoid pushing the envelope too far.

The spacecraft started rolling rapidly 50 s into the burn, while travelling at Mach 2.7. This was probably due to, or at least exacerbated by, pilot error. The pilot was not highly concerned by this, being confident that he could correct the situation, and he allowed the burn to continue during the roll. He later said "I thought it was kind of cool".

The apogee altitude was estimated by the nearby Edwards Air Force Base, based on radar data, to be 337,569 feet (102.9 km). Due to the early burn cutoff, this was far less than originally anticipated.

Overall, the craft did 29 complete rolls. Atmospheric reentry proceeded normally, with the craft rapidly righting its attitude due to the stable high-drag configuration. It changed back to gliding configuration normally, glided back to the spaceport, and landed safely at 08:34.