The first M7s produced were modified M3 Lee medium tanks. In order to maintain a low silhouette, the howitzer elevation had to be restricted to 35°. In May 1942, after only a month of production, the vehicle was altered to increase its ammunition stowage from 24 to 69 rounds. This was achieved by placing seven rounds on the left wall, five on the right, and storing the remainder under floor plates.
he M7 also went through a fairly rapid shift from being based on the M3, to having more commonality with the M4 Sherman. The first major example was an adoption of the M4's three piece housing, single piece casting and suspension. In British service, some M7s carried a radio set, which took the place of 24 rounds of ammunition.
