developed beginning in October 1941. It was an M3 Lee chassis mounting a 3-inch gun M1918 in a limited-traverse mounting. The T24 was initially rejected as being too tall and the T40 was its improved derivative. The T40 was standardized as the "M9 3-inch Gun Motor Carriage" for limited production in May 1942 but never built. Meanwhile, as the final design developments of these two tank destroyers were underway, the Ordnance Department became dissatisfied and by November 1941 had issued an additional specification for a tank destroyer with a 3-inch gun in a rotating turret. Design work began immediately. The 3-inch Gun Motor Carriage T35 combined an early-production M4A2 medium tank hull with the 3-inch gun M7 from the M6 Heavy Tank in a cast, circular, open-topped turret. Using lessons learned from combat reports from the Philippines, the armor on the sides and rear of the upper hull was changed from flat to sloped plates. This new test vehicle was designated the 3-inch Gun Motor Carriage T35E1. Prototypes of these two vehicles were delivered to Aberdeen Proving Ground in April 1942, and the Army selected the T35E1 for further development on 2 May 1942. The side and rear upper hull armor of the T35E1 was thinned from 1 inch (25.4 mm) to 0.75 inches (19 mm) in order to reduce the weight of the vehicle. The staff at Aberdeen Proving Ground was worried that the armor of the T35E1 was too thin, and so bosses for appliqué armor panels were added to the hull sides, glacis, and turret sides. As the cast turret was found to be difficult to manufacture, the design of the turret was changed to a sloped pentagonal shape made of welded armor plate. In June 1942, the modified T35E1 was standardized as the 3-inch Gun Motor Carriage M10. By fall 1943, with its M5 and M9 competitors being eliminated from the design competition and their production contracts cancelled, the M10 was to become the United States' primary tank destroyer of the early war period. As there were fears that the production of M4A2 chassis would be inadequate for M10 production, an alternate design, the 3-inch Gun Motor Carriage M10A1 based on the M4A3 medium tank chassis, was also authorized for production. Design American tank destroyer doctrine emphasized speed and gun power over armor. As a result, the M10's armor was thin, which made it vulnerable to