The Ball is a cheap combat vessel created to supplement the Earth Federation's limited numbers of GM mobile suits. While planning Operation Star One, the massive space campaign to take the war into Zeon's home territory, the Federation decided to refit approximately 1200 civilian-model SP-W03 space construction pods with armor plating and a 180 mm recoilless cannon. Despite their limited firepower and low maneuverability, the Federal Forces would deploy this RB-79 Ball as mobile fleet defense units, protecting the Federation's ships while the GM mobile suits went on the attack.

Restoring the previously-discarded RX-76 development plan, the Federation quickly began production of the RB-79 Ball. A limited number of RB-79K trial units were created, armed with a twin cannon turret, but the final production version sported a single 180mm cannon. About 1,200 Balls were used in Operation Star One, where they provided medium-range fire support for close-combat GM teams.

Source: Gundamproject.com, Mobile Suit Gundam