The Holt tractor company was the largest manufacturer of tracked vehicles at the outbreak of WW1. Both Britain and France bought Holt tractors to tow artillery pieces in the early years of the war. These proved to be very successful since they permitted the movement of heavy guns in conditions which would have been impossible for the traditional horse transport. The U.S. Army bought Holt artillery tractors in 2½, 5, 10, 15 and 20-ton towing capacities. The 15 and 20-ton tractors were Holt commercial vehicles minimally modified for military service. However, the smaller tractors were specially designed for the U.S. Army by Holt.
At the entry of the U.S. into WW1 some 24,791 tractors of all types were ordered. By the time of the Armistice in Nov 1918 2,500 had been delivered and more than half of these were shipped to France. The obvious requirement for tracked support vehicles was not ignored and the design of tracked munitions carriers and petrol tankers was well advanced in Nov 1918.
The Ordnance Department designed SPGs based on the Holt 2½ and 5-ton artillery tractors as gun carriers at the same time as the Rock Island Arsenal was building SPGs based on the 10-ton tractor. Very little information has been published on these vehicles but some observations may be made from surviving images of these vehicles.
The gun used was the 3 inch M1916 gun - an advanced US-designed light field gun intended to become the standard field gun of the U.S. Army. However, the French 75mm Mle 1897 built in the U.S. as the M1897 was chosen as the standard field gun and the M1916 was discarded.
The 2½ ton tractor was fitted with an M1916 gun by extending the side plates of the tractor chassis to support trunnions in which the gun axle fitted. The excess axle length was cut off. There were a pair of outriggers fitted at the rear to stabilize the vehicle during firing since the short length and high, rearwards position of the gun would have tended to topple the vehicle.
