On August 29, 1940, (the day after the M3 was decided to be put into production) work began on a tank that would mount a 75 mm gun in the turret.
In March 1941 the turret was designed based on the turret on the M3 Medium. The Armored Forces Board selected the simplest design out of 5 options in April 1941. A mockup of the T6 was approved in May 1941. Was standardized in September 1941 / October 1941. The design was influenced by the Canadians and British as it had a resemblance to the Canadian Ram.
The driver sat on the left in the front of the hull, and the assistant driver on the right. In the turret the loader sat on the left of the main gun, the gunner on the right, and the commander at the rear behind the gunner. Adjustable seats that could move 12 inches up and down, and 5 inches forward and backwards were provided for the driver, assistant driver, and gunner. There were 2 hatches in the top of the hull and a revolving hatch in the turret. There was a hatch installed in the floor, behind the driver, for emergency escapes.
Each member of the crew was provided a periscope. They could be rotated 360°, and tilted up and down. The gunner's, which contained a telescopic sight, was synchronized with the main gun. Early models had direct vision slits that were protected by thick glass plates and hinged covers for the driver and assistant driver. After experience with bullet splash these were eliminated and replaced by periscopes.
The commander had a periscope in the turret hatch that could be used when it was closed. Starting in 1944 it was replaced by a cupola with six episcopes.
There was also a 2-way radio and interphone system. They were located in a shock mounted shelf in the turret bulge.
The transmission had 5 forward and 1 reverse speed. It also had a parking brake built in. The controlled differential transmitted the power to the final drive unit, and also contained a brake system for steering and stopping the tank.
In the engine compartment were 2 fixed 10 lb. fire extinguishers. They could be operated from the driver's seat or from the outside. There were portable 4 lb. fire extinguishers in the turret and driver's compartment.
Auxiliary generator provided extra power and could be used in preheating engine in cold weather.
There were six 2-wheeled bogies bolted to the hull which supported the vehicle on volute springs. The drive sprocket was located in the front. The idlers were at the rear and could be adjusted to take the slack out of the tracks. There were 3 return rollers supporting the weight of the track. Many of these rollers were above the suspension brackets, but other models had them shifted towards the back with a track skid on top of the brackets.
In 1944 the Horizontal Volute Spring Suspension (HVSS) was phased into production. It enhanced mobility.
The driver used levers, which operated steering brakes, to steer the M4.
The early vehicles had the Combination Gun Mount, M34, which had a shield that only protected the gun. In October 1942, the Ordnance Committee chose to use the Combination Gun Mount, M34A1. This had a shield that also protected the .30 cal coaxial machine gun and telescopic sight. It also had 2 pieces of armor that were placed beside the gun, where it met with the shield.
The tanks had azimuth and elevation controls installed so that the main gun could be used an artillery weapon. Also FM communication radio systems were installed and could be tied into any local field artillery fire control. It could often run 2,500 miles before major maintenance on the tracks and bogies.
Early models had reputation for "brewing-up" (nicknamed Ronsons after a cigarette lighter) when penetrated by antitank rounds. Water jackets (signified by W in model name) were installed around the ammunition and on howitzer models additional armor was added. The water jackets used 38 gallons of water, that was mixed with antifreeze, and ammudamp (anti-corrosion).
Turret rotated on ball bearing race that was recessed and thus protected from enemy fire. The 75 mm gun was turned 90° from vertical, which allowed for easy right-hand loading.
There was a elevating hand wheel for the gun. If the gyrostabilizer was activated, then hydraulic power kept the gun steady while on the move.
The gunner had foot operated switches that fired the guns electronically.
Comparison of Main Tank Armament Performance