In the fall of 1941 Cadillac suggested to the Ordnance Department that they should try the M3 with a twin Cadillac engine due to radial engine shortages that were used by the aircraft industry. It was also to have the Cadillac Hydra-matic transmission that was used in automobiles.
In February 1942 an M3A1 had 2 Cadillac liquid cooled V-8 car engines installed which meant that the rear deck had to be raised. Fuel tanks were put in the rear corners of the hull. Radiators were placed above the engines.
The flywheel in each engine was connected to the Hydra-Matic Transmission. Since the drive shaft from the Cadillac engines and the Hydra-Matic transmission, some of the traversing mechanisms and parts of the gun stabilizer were moved under the turret basket and this provided more room for the commander and gunner. The turret was extended in back to accommodate the radio.
A single cylinder engine supplied auxiliary power and charged the batteries.
The seats for the driver and assistant driver could be locked in any position. The seats went up under spring pressure and down under body weight. The driver and assistant driver had 360° periscopes in the roof of the hull.
Four escape hatches were provided.
It was originally going to be designated the M4 Light Tank, but it was decided to use M5 to avoid confusion with the M4 Medium.
The M5's hull was welded armor plating with the front plate reinforced. The turret was also welded.
The M5's 37 mm M6 gun could fire an APC shell at a muzzle velocity of 2,900'/sec. It's range was 12,850 yards. At 1,000 yards the M6 gun could penetrate 1.8" of armor.
There was a gyrostabilizer which helped keep the gun at a fixed elevation during movement.