The Medium Tank M4A4E1 was the first prototype for the 105mm howitzer armed versions of the M4 Sherman.
When the Sherman was first being designed, five different combinations of turret armament were considered, and the turret front plate was designed to allow the use of any of them (with the correct mount). One of the combinations suggested was the use of a 105mm howitzer with a coaxial .30in machine gun.
Work on producing a 105mm armed version of the Sherman began in 1942. The first task was to produce a suitable gun mount, the combination mount T70. This had a 3in cast gun shield and could use a direct sight telescope. The two mounts were ready by November 1942, and were installed in two M4A4s, which then gained the designation M4A4E1.
Tests on the M4A4E1 began at Aberdeen on 7 December 1942, and at the Armored Board at Fort Knox soon afterwards. These uncovered a number of problems with the howitzer installation, including with the firing mechanism, the recoil system, the layout of the breech and the balance of the turret. As a result the howitzer had to be modified, and the new version was approved in February 1943. The power traverse mechanism, which struggled with the unbalanced turret, was removed, and a partial turret basket with seats for the three turret crewmen was added. These modifications were then tested out in two pilot vehicles, the Medium Tank M4E5.