The NX-Beta on an unathorized flight in 2143.
The NX-Beta, along with the NX-Alpha, was one of two test vehicles for used by Starfleet's NX Program with the goal of breaking the Warp 2 barrier. In 2143, Jonathan Archer and A.G. Robinson took the NX-Beta on an unauthorized flight shortly after the destruction of NX-Alpha. The vessel was able to reach a maximum speed of warp 2.5 and return to Earth safely, the first vessel to do so. (ENT: "First Flight")