The first Earth Federation mobile suit developed under their top-secret "Project V", the RX-75 Guntank was hardly a mobile suit at all. Developed four years prior to the outbreak of the One Year War, the Guntank was nothing more than a fusion of a tank with the upper torso and arms of a mobile suit, and was originally intended to serve as an anti-MS unit when rumors of Zeon's first mobile suits reached the Federation. Despite this apparently faltering first step, the Guntank packed a serious punch as a heavy fire support and artillery unit. The Guntank was also somewhat unique in that its piloting and gunnery requirements were originally split up between two cockpits: a gunner's station in the head, and a standard cockpit in the torso. The Guntank, like its Project V brethren RX-77-2 Guncannon and RX-78-2 Gundam, featured the core block system, which allowed the Guntank's upper torso to be ejected to continue operating as a fixed gun emplacement while the torso pilot ejected the core block to keep fighting in a FF-X7 Core Fighter.

The first RX-75s were deployed for testing and trials at the Federation's research base on the Side 6 colony. However, only one unit - the RX-75-4 - survived an attack when Project V was discovered by Zeon forces in September UC 0079. This last unit was transferred to the assault carrier White Base, where it operated as a support/defensive unit until it was decommissioned at the Federation's Jaburo military headquarters in November UC 0079. During its service aboard White Base, the Guntank was upgraded so that one pilot could handle both piloting and gunnery functions from the head cockpit.

Pilot(s): Ryu Jose, Hayato Kobayashi, Kai Shiden, Amuro Ray

Original mechanical designer: Kunio Okawara