T49 GMC (M18 Hellcat prototype)

This American self-propelled tank destroyer served as the prototype for the M18 Hellcat. 

By the summer of 1942, the company Buick produced the first T49 GMC. Testing began that July. But by that time, command required a more powerful weapon, and requested a second experimental model, now with a 75mm M3 main gun, borrowed from an M4 tank. 

As a result, the T49 project was abandoned in favor of the modification with the more powerful weapon, and a second experimental self-propelled gun designated the T67.