Earth's first prototype for an outer space combat vessel, intended to serve as a platform from which Earth could be defended from attacking ships. The X-301 Interceptor was a retrofit of two of Apophis's death gliders, armed with two A.I.M. 128A air-to-air missiles, each carrying naquadah-enhanced warheads.
If the tests had been successful, the ship was to be assigned to orbital defense under the auspices of Stargate Command. After the intial test flights failed and the X-301 was lost, the program seems to have been abandoned in favor of the X-302 and X-303 projects.
Touchstone - SG-1 gets a peek at the death gliders under study at Area 51, where the X-301 is in development.
Tangent - During its intial test flight, an embedded Goa'uld recall device forces the X-301, carrying O'Neill and Teal'c, to alter course toward Apophis's homeworld.