M4 Tractor Wikipedia
The M4 was based on the chassis and drive train of the obsolescent M3 Light Tank which introduced the trailing idler. One variant was designed to tow the 3inch or 90 mm antiaircraft gun, and another was for the 155 mm gun or 8inch howitzer. The rear compartment carried the gun crew and other equipment and some later variants included a crane to assist with heavier projectiles.