The M4 was based on the chassis and drive train of the obsolescent
M3 Light Tank which introduced the trailing
[2] idler. This common practice of re-using old vehicles simplified design, allowed for easy production, and made maintenance in the field easier.[
citation needed]
One variant was designed to tow
anti-aircraft guns and another for
howitzers.
[1] The rear compartment carried the gun crew and other equipment and some later variants included a crane to assist with heavier projectiles.
[1]
Two types of ammunition boxes were used on all models: a
90 mm box with side "tailgates" to access 90 mm shells pigeon-holed in the sides, and a combination box for
155mm /
8-inch /
240 mm ammunition with a rear tailgate, and hoist.[
citation needed]