This tank is accurately described by the saying "third time's the charm". When the American National Armament Program started in 1940, the US had no medium tank that could be mass produced. The program declared that the United States must produce 14.5 tanks per day by the end of 1940, even though it was not clear which tank. The existing M2 Medium tank was a weak candidate due to its 37 mm gun. 92 M2A1 tanks were built from January to August of 1940, exclusively as a temporary measure until a new medium tank began mass production.
So, the 37 mm gun was insufficient for the Army. Infantry commanders demanded that the new tank have a gun at least 75 mm in caliber. The problem needed to be solved quickly, but the American engineers had no turret that could fit this kind of gun. In order to save time, the engineers decided to make a poor move and build a wooden mockup of the tank with a 75 mm gun in a sponson on the right side of the tank. This stroke of "genius" greatly complicated the lives of American tankers, since it made aiming in a 360 degree range impossible. The tank would have to spin like a top.
In the engineers' defense, they also did not agree that this was a good move, and admitted that it would only be a temporary design, until a tank with a turret that could fit a 75 mm gun was designed. The Army decided that about 350 tanks would be built, then production of new tanks with a fully rotating turret would start.
The issue of tank manufacturing was a painful one for America. It did not have the appropriate manufacturing facilities. There was only a small government factory at Rock Island Arsenal, incapable of satisfying the increasing demand. Private manufacturers needed to be involved. The choice was between approaching heavy machinery manufacturers, or automobile ones. The decision was made towards the latter, as heavy machinery largely deals with one-off designs, and car manufacturers were used to true mass production. The government offered Chrysler to share the costs of building a tank manufacturing plant in Michigan. The government would be the owner of the factory, but Chrysler would be in control of it. Close collaboration with Rock Island Arsenal would provide compatibility between the various technologies and devices in the future tank.
Aberdeen's engineers started developing the M3. The new tank had a similar engine as the M2, and the same suspension. Homogeneous rolled armour was attached with rivets, like on the M2. The turret and sponson were cast. In order to reduce spalling, the inside of the tank was covered in porous rubber.
The tank crew was initially 7 people. They had to get into the tank through side doors, sponson hatch, and the commander's cupola. It gave the 31 ton tank very good visibility.
By February of 1941, the new tank project was ready, and the Michigan factory was almost complete. All that remained was to build the new tank and test it. A sample was sent to Aberdeen on March 13th, 1941. Test results were unsatisfactory: poor crew compartment ventilation, vulnerable side doors, high chance of the gun getting stuck in the sponson after taking a hit, weakness of the suspension. However, the turret controls and gun stabilizer were excellent; the loader could aim very comfortably even when zigzagging across harsh terrain.
The next M3 had all of its doors replaced with an evacuation hatch on the bottom, the crew was reduced by one, the periscopic sight was replaced with a telescopic one, among many changes. In August of 1941, the M3 began mass production. From August of 1941 to December of 1942, over 3500 M3 tanks of this type were built.
The British also used M3s. They named their tanks "Grant", and the American ones "Lee", after American Civil War generals.
As we said before, the M3 was built only because nothing better existed. A large portion of the tanks were sent to Britain and the USSR. The latter received 976 vehicles, spread across independent tank battalions, brigades, and regiments. The American tank fought on all fronts, was used at Kursk, and one even made it to the Far East. The Red Army was not a big fan: the tank had poor cross-country performance, was very tall, and had rubber tracks that burned up easily. The tank became immobilized, and presented a nice target for enemy guns. The tracks also frequently fell off. Many complaints were made about the 75 mm gun, which was very difficult to aim. These drawbacks led to the M3's nickname in the Red Army: BM-6 (Bratskaya Mogila - 6, Grave for Six Brothers).
In the armies of the Allies, the M3 was completely replaced with the Sherman by 1944, and the Soviets got rid of theirs as fast as they could. However, after the war, they still saw use in South-East Asia. Many vehicles were developed on the M3 chassis, from SPGs to engineering vehicles.
Original article available here.