With growing tensions between the Empire of Japan and the United States heading into World War 2 (1939-1945), the American military was in dire need of modern war-making systems. This was painfully clear in the department of anti-armor solutions which spurred development of a make-shift self-propelled vehicle mounting the 3" Anti-Tank (AT) gun (essentially the World War 1-era M1918 anti-aircraft gun) to the existing chassis and hull of the M3 Lee/Grant Medium Tank. The M3 was the principle American frontline tank heading into the war and its useful chassis went on to form the basis for many vehicle forms as the war progressed and operational service shifted to use of the new M4 Sherman line in 1942.
The new tank destroyer became the 3" Gun Motor Carriage (GMC) T40 which was to carry the in-service designation of "M9". This vehicle retained all of the form and function of the M3 tank though with a revised, open-air, octagonal superstructure added to fit the length and height of the 3" gun. The gun also came with its own mounting hardware, elevation/traverse controls and recoil mechanism but was largely dependent upon the entire vehicle being pointed into the direction of fire. The driver sat in the hull at front-left with the gun unit protruding from the front shield at center. The M3 hull superstructure was of a decidedly lower profile in the T40 which was a good battlefield quality to have. The running gear retained the rear-mounted engine, the front drive sprockets and the rear track idlers. Three double-wheeled bogies were situated along each hull side.
The Baldwin Locomotive Works was charged with modifying the M3 hulls to fit the T40 design standard and this work began in 1941. Some 1,000 of the type were on order by the American Army following the Japanese attack on Pearl Harbor in December of 1941. Despite the work ongoing through much of the early part of 1942, the program was cancelled during August due to the limited stock of 3" guns on hand as well as the shifted in focus to the promising M10 "Wolverine" tank destroyer. This left the T40/M9 project to the pages of history and nothing more.
The M10 would go on to become a classic American wartime fighting vehicle with over 6,700 produced from 1942 to 1943.
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