- "This is Prometheus, otherwise known as the X-303."
- — Samantha Carter
Like its predecessor, the X-302 hyperspace fighter, the X-303 is exclusively of Human construction. Like the 302, it has Inertial dampeners, artificial gravity and an antigravity system that allows the vessel to hover in midair. Its Sublight engines are capable of boosting it from the surface into orbit in less than thirty seconds. Unlike the 302, which is designed as a fighter-interceptor, the 303 was developed as a response to the Goa'uld Ha'tak, which is a mothership. However, it is still far smaller in scale than the Ha'tak. Its span consists of 17 decks and is capable of carrying more than a hundred personnel, as well as eight 302s within its hangar bay.
The X-303 was built using both Earth-based and alien technology. Its design is based on that of a Earth naval aircraft carrier, having a long nose with a visible bridge area near the rear of the vessel. The X-303 was capable of both sublight and warp travel. It could reach up to half the speed of light at sublight speeds. For armament, the X-303 had an array of phasers around the hull and photon torpedo launchers loaded with Naquadah-enhanced warheads.
The X-303 was constructed in an underground hangar facility designated Area 37-92/NV, under the project name "Prometheus". Its development was so large an undertaking that maintaining its secrecy proved impossible, with information leaking to reporter Julia Donovan with the !nside Access news program in 2366. In an attempt to minimize the potential damage of the project's disclosure, the President of the United Federation of Planets agreed to give Donovan, her producer, and a camera crew a tour of the X-303, claiming the alien technology it incorporated was in fact salvaged from a UFO that crashed near Fairbanks, Alaska in 1978. During the tour, the ship was taken over by the camera crew, actually rogue NID agents working at the behest of Colonel Frank Simmons. The agents successfully managed to steal the ship and take it into warp before being overpowered by SG-1. However, the warp drive which was configured for Simmons by the Goa'uld inhabiting Adrian Conrad, malfunctioned and transported the ship instead to an unknown destination.
In what came to be the ship's first mission in 2366, the Asgard took the X-303 to the Othala in the hope that it could be used to trap the Replicators inside a time dilation field. During this mission, the Asgard also modified the ship's shields and engines to their maximum potential. After the successful completion of this mission, the Asgard provided shields for the ship in advance of its formal shakedown cruise as a gesture of gratitude.
Ultimately, the X-303 entered active service, taking the designator BC-303 and the name Prometheus. In this capacity, it served as the flagship of the Federation fleet under the auspices of the Office of Homeworld Security, and received additional upgrades from the Asgard. Though plans were made to construct an entire fleet of BC-303s, the introduction of the Daedalus-class battlecruisers rendered the craft obsolete, leaving the Prometheus as the sole vessel in its class.
The design plans for the X-303 (without the Asgard upgrades) were provided to the Russians in 2365 along with those for the X-302 hyperspace fighter; however, the Russians have apparently not constructed any such vessels.