Half-tracks were developed for the US Army from the early 1930s after the US Ordnance Department had studied the successful Citroen-Kegresse half-tracks. Various firms in USA developed half-track designs for the Ordnance Department, the Gunningham Motor Company produced the very first, designated T1, in 1932.
By 1940 development had led to the Half-track T14 after which the appearance of all subsequent WW II period American half-tracks was finalised. Later in 1940, with Europe at war, the USA commenced a big rearmament programme, and in September 1940 the existing T14 was standardised and ordered into production as the Half-track M2.
As originally conceived, the half-track was seen as an artillery prime mover, the M2 being equipped to tow the 105 mm field howitzer and its crew and ammunition. The very successful use of the Hanomag half-track (Sd Kfz 251) in the armoured infantry (Panzer-grenadier) role by the Germans in Poland in 1939 and the French campaign of 1940, led the US Army to realise that the half-track offered a wider range of role than that of an artillery mover. The US Army Staff studied the German campaigns, America not then at war, so when the big American military expansion started in the latter part of 1940 several projects were influenced by lessons learned from the victorious German campaigns.
To adapt the existing T14/M2 for use as a personnel carrier, slight modifications were made. Inward facing seats for 10 men, stowage for arms and equipment, a body 10 inches longer and a rear door resulted in the Half-track Personnel Carrier M3 which became better known, and more widely used than the original M2. The M3 became standard for armoured infantry throughout WWII and was used by other nations for years afterwards.
Within the infantry battalion were the mortar platoons, and a further half-track adaptation was needed to transport the 81 mm Mortar M1.
The earliest mortar carrier was the Half-track Mortar Carrier M4, standardised from October 1940. This was simply the basic M2 with revised stowage to allow the 81 mm mortar to be carried (facing to the rear) with lockers for ammunition, plus crew seats. The vehicle was intended to transport the mortar which was fired from a ground position, though it could be fired from the vehicle in an emergency only. The disadvantage of this vehicle was the lack of traverse facility for the mortar mount, the lack of chassis reinforcement, and the fact that the weapon faced aft.
A modified Mortar Carrier M4A1, having a chassis suitably stressed for the mortar to be fired from the vehicle was started in 1942. Meanwhile a revised design, based on the M3 personnel carrier, was produced under the designation Half-track Mortar Carrier M21. This had the longer M3 body with rear door, the mortar now arranged to fire forwards with a special traversing mount.