The X-303 was an experimental human-built craft designed as a countermeasure to the Goa'uld mothership. The following year when the craft became a part of Earth's fleet, it became officially known as the battle cruiser BC-303. Prometheus was the first in the series of 303 battle cruisers, and General Vidrine at the Pentagon was put in charge of BC-303 production. The 303 series of spacecraft was eventually replaced by the more advanced BC-304 series which included the Daedalus, Odyssey, and Korolev.
Cross Reference: BC-304, Naquadah, Prometheus, General Vidrine, X-303
Episode Reference: Prometheus, Unnatural Selection, Memento, Grace, Lost City, New Order, Endgame, Prometheus Unbound, Full Alert, Avalon, Beachhead, Ex Deus Machina, Ripple Effect, Ethon