had started to come off the production line, the M3 Lee. Devers observed testing of one of them just days after taking command, and he was unimpressed. American industry was unable to cast a turret large enough to hold a 75mm gun and so the M3 carried a 37mm gun, with a 75mm in a sponson. That gave it a limited traverse and made it difficult to engage moving targets. To make matters worse, the designers shortened the barrel to improve the M3's mobility, which Devers realized also reduced the gun's muzzle velocity and hence its effectiveness against armor. In practice, M3 crews attempted to engage German armor with the 75mm, as the 37mm was ineffective against it. It was found that the 75mm could penetrate the frontal armor of German tanks at , but German tanks destroyed M3s at up to . Devers pronounced the M3 "overweight, underpowered, and insufficiently armed." Often against the views of his superiors, Devers lobbied for a still more heavily-armored and better-armed medium tank, the M4 Sherman. He played an important role in the M4's design, development and manufacturing, particularly its engine and armament. Throughout his tour as Chief of the Armored Force, he worked closely with the United States Army Ordnance Department, manufacturers and the Armored Force Board at Fort Knox on the research and testing of tanks, guns, armored vehicles and ammunition. The biggest obstacle was engines. Those of prewar tanks were rated at , insufficient for a medium tank. Devers wanted an engine, but that was beyond the ability of the American automotive industry to produce. Extraordinary efforts resulted in the development of a number of engines. He controversially rejected the General Motors 6046 diesel engine in favor of a gasoline engine. Battlefield experience would demonstrate that the diesel engine was superior. The quest for a better engine eventually settled on the Ford GAA engine, but there was a persistent shortage of tank engines. Some 49,234 of the reliable, versatile, low-cost M4 Sherman and its variants would be produced. After the GHQ Maneuvers, the Army expected to have a period of "remedial training" to fix problems. The Japanese attack on Pearl Harbor on 7 December 1941 shattered those expectations and plunged a not fully-prepared United States into the war. In early 1942, two armored divisions were operational and three were in training: the 6th Armored Division was activated in February, followed