Even as far back as 1915, the U.S. Army Ordnance Department was investigating the mechanization of field artillery beyond simply using tractors to move artillery pieces about the battlefield. During World War One, the Holt Tractor Company was the largest producer of tractors and supplied not only the U.S. Army but also the armies of Britain and France with all three nations using the tractors to pull their respective heavy artillery pieces across the battlefield where horses and trucks could not tread. It would be the British that beat the U.S. to the punch with the development of the Gun Carrier Mk. I, the world’s first production self-propelled gun. Using components from the original British Mk. I tank, the fully tracked Gun Carrier Mk. I was designed to carry either the Ordnance BL 6in. howitzer or the Ordnance BL 60-pounder gun. The vehicle was meant to move the guns to firing positions where they would be dismounted and set up but if need be, the 6in. Gun (not the 60-pounder) could be fired while still on the Gun Carrier. Deliveries of the Gun Carrier Mk. I started in mid-1917 and in all, some 48 vehicles were delivered by the end of the war. As it was, in practice, the Gun Carrier Mk. I was used more as a supply vehicle and rarely as a self-propelled gun. Two major occasions where the Gun Carrier Mk. I functioned as a self-propelled gun was during the Battle of Pilckem Ridge (that ran from July 31, 1917 to August 2, 1917) and the Battle of Amiens (from August 8 through August 12, 1918) and in both cases, the crews fired the howitzers then shifted positions in order to avoid any German counter-battery fire. This is a tactic used even today by self-propelled artillery.
Returning to the U.S., the Ordnance Department was looking to mount the new 3in. M1917 anti-aircraft gun on a tractor and the Holt Tractor Company stepped up to the plate and suggested a proposal by Pliny E. Holt for just such a mount. Authorization was received to commence development and construction of a prototype and sometime in 1917, the all caterpillar tracked Holt 55–1 was completed and submitted for testing. The Holt 55–1 was a relatively simple affair, consisting of a rectangular riveted metal chassis with the engine, fuel tank, and driver’s station in the rear and the M1917 gun fitted to a high-elevation pedestal mount in the center of the chassis. There was no armor or shielding to protect the power-train, crew or gun though the 55–1 did have outriggers that were deployed to stabilize the vehicle during firing. Testing showed that the 55–1 did have some problems but nevertheless, it demonstrated that the concept of a self-propelled artillery gun was worthwhile. The 55–1 was refitted to carry the heavy Vickers built BL 8in. Mk. VIII howitzer and that is what is shown in this picture distributed by the news photography division of Underwood & Underwood. The mount for the howitzer was made of thick oak planking and does not appear to have had any elevation capability. However, as it was simply a test mount, the Holt 55–1 performed adequately, being able to absorb the recoil of the howitzer without any difficulty. Like the previous gun mount, the revised Holt 55–1 had two extendable outriggers on each side though in this retouched photograph, they appear to be missing. Very few actual photographs exist of the 55–1 and those that do don’t show any camouflage paint applied. In addition to the two gun mounted variants, the 55–1 was also tested as a unarmed (and unarmored) transport, the driver’s station having been moved to the front of the vehicle. The Holt engine along with it’s radiator can be seen with the driver sitting in front of it. On the other side of the driver is the fuel tank for the engine. It appears that there is another man with steering levers on the front of the 55–1 but he is likely just holding on to the outrigger components as there was no room for a driver station in front with the gun mounting. Thus, the driver worked both the speed control as well as the steering levers for the vehicle. The driver actually had no seat and instead, he is sitting on the oak planking that went around the top of the vehicle. Likewise, the three standing men are also atop the oak planks, one of them holding on to a metal brace supporting the gun. It is unknown what the crank is for as an original photograph of the 55–1 doesn’t show such an apparatus so it may have been added to the photograph.
The 55–1 weighed 11 tons, was 20.6 feet long, 10 feet wide, and 9.5 feet high. Power came from a single Holt 4-cylinder engine that developed 75 horsepower. The engine was tied into a Clark 5-speed transmission. This gave the 55–1 a maximum speed of 5mph. The 8in. Mk. VIII howitzer fired a 200 pound high-explosive shell at a muzzle velocity of 1,500 feet per second. This gave the gun an effective range of just shy of 7 miles. The normal elevation of the gun was 0 degrees to a maximum of 45 degrees and a traverse of 4 degrees to the left or right. The breech was of the Welin interrupted screw type with a Asbury Breech Mechanism that opened the breech by combining the unscrewing, withdrawing, and swinging clear movements of the breech into one continuous action. Likewise, the closing of the breech after the round and charge was loaded was also one action. This sped up the loading and unloading of heavy guns which allowed for a increase in the rate of fire. Recoil was absorbed by a hydro-pneumatic recuperator and hydraulic buffer.
Only one Holt 55–1 was constructed and it was not put into production. Pliny Holt was lured away from the Holt Tractor Company and went to work for the Rock Island Arsenal, Rock Island, Illinois and while there, he oversaw the development of a series of self-propelled guns, the Mark I through the Mark X between 1918 and 1922. Fifty Mark I (with the 8in. Mk.VIII howitzer), fifty Mark II (with a 155mm M1918 gun), and a total of 250 Mark III and Mark IV (with a 240mm Schneider TR M1914 howitzer) self-propelled guns were contracted but the end of World War One saw the contract drastically cut back to only a handful of vehicles for experimentation use only. Despite none of Holt’s vehicles making it into production service with the U.S. Army, they did provide significant knowledge in design that would come into play when war once again loomed on the horizon.
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