Hitler believed that the American economy, while formidable, would not be an obstacle as it was in a peace-time state, he believed it would be years before the Americans were able to re-tool and field an effective force. This was not the case. Overy contends that the socio-economic planning experience gained by the U.S. government during Roosevelt's New Deal was important to the re-tooling. He also cites an American "can-do ethos" as a spiritual driving force behind the re-tooling. Finally, the very fact of the Great Depression had left the American economy in a position where it had a lot of room to grow. The German economy, by contrast, was already operating at full employment levels by the time of the war's start. The final factor Overy cites is the success of Allied air power. Allied bombing campaigns in Europe were effective in stifling German war-production and also in demoralizing the German populace. The success of Allied air operations behind German lines also forced the...

While in America factories chugged along un-molested, in Europe the Germans were forced to de-centralize production and spend vast resources on improbable projects for safeguarding factories.
Overy admits that "there are many other factors that explain victory and defeat" in WWII, but makes a strong case that the adaptive ability of the Red Army and the American economy coupled with Allied air power were the overriding causes. In the Professor's own words:

There were weaknesses and strengths in Hitler's strategy, but no misjudgments were more costly in the end than the German belief that the Red Army was a primitive force, incapable of prolonged resistance, or Hitler's insistence that the U.S.A. would take years to rearm and could never field an effective army, or the failure to recognize that bombing was a threat worth taking seriously before it was too late.