Originally designed in 1935 as a civilian project to enable easier crossing of swamps that would be otherwise inaccessible, and attracted the interest of the US Marine Corps.
Re-designed as the US entered WW2 200 were initially ordered and could carry 18 men or 2000 kilos of cargo. But as they were considered 'transport' they had no armour, initially at Guadalcanal they were used to ferry troops and supplies from ship to shore and took little direct role in combat.
But soon they were being armed with machineguns and some 'field modifications' of armour, some 7000 being built by the end of the war and being heavily engaged in the island hopping operations.