The United Earth Starfleet programme of the 2140s that flew the NX-Alpha and NX-Beta test craft past warp two, clearing the way for the warp five ship Enterprise.
The NX Project, also called the NX programme, is the series of test flights by which Starfleet proved Henry Archer's warp engine design. It is shown in flashback in the Enterprise episode First Flight.
Under Commodore Forrest, the pilots competing to fly the first craft include Jonathan Archer and A.G. Robinson. Robinson is chosen and takes the NX-Alpha beyond warp two, but pushes on and the ship is destroyed, though he ejects safely. The Vulcans advise that the engine is unsound and Starfleet suspends the work.
Archer, Robinson and the engineer Charles Tucker, meeting here for the first time, take the NX-Beta without permission and reach warp 2.5, proving the fault can be corrected. The programme resumes, leading in time to the launch of Enterprise NX-01.