Adventure Comics, Vol. 1 #422 (1972)

Pawn Of Peace! / Rodeo Of Death
"Pawn of Peace": Two alien invaders with the power to make themselves look like Earthmen pose as scientists from a "peace committee" and pay a visit on Professor Axel. Axel has spent ten fruitless years trying to make a giant warrior-robot work. The aliens lend a hand and animate the robot with their superior technology. But they intend to use it as a weapon, to wreak havoc in San Francisco. On her way to the job Linda Danvers is struck by a car tossed by the robot. Linda pretends to be unconscious and changes into Supergirl as soon as possible. 
Release Date: 1972