Military aircraft production represents a (somewhat) reasonable proxy for overall military production. In 1940, military aircraft production was as follows:
A large portion of the American aircraft production was being sent to Britain, through lend-lease and other means. Moreover, British and American leaders had made plans to expand American military aircraft production to the staggering total of 72,000 planes per year, with half of that production going to Britain. Those plans placed significant intermediate-term pressure on Germany; and created a strong incentive for it to increase its access to manpower, raw materials, and industrial capacity to counter the Anglo-American threat to German cities and the German population. Hitler hoped to conquer the Soviet Union before the Anglo-American air threat had really kicked in. At that point, he’d use the resources gained from that conquest to even the odds in the air war.
In 1941, the US increased its aircraft production to 19,000 planes, even though it was still technically at peace until December of that year. By 1943, it had increased military aircraft production to 86,000 per year; surpassing the goal American and British leaders had set back in 1940. If that American military aircraft production could have been kept neutral (as opposed to being sent to the British and Soviets under Lend-Lease), it would have radically altered the strategic equation.
In 1942, military aircraft production was as follows:
Those numbers would seem to spell doom for the Axis, even if American military aircraft production could have somehow been removed from the equation. However, if American industrial capacity had remained neutral (as opposed to being sent to the Allies), Hitler would have had the option of waiting to invade the Soviet Union. That would have allowed Germany to consolidate its position in Central and Eastern Europe, while building up its industrial capacity. By 1944, increases in military productive capacity had led to the following aircraft production:
It’s also worth noting that in 1942, Germany produced 4,000 tanks (excluding light tanks) to the Soviets’ 15,000 tanks (excluding light tanks). That difference was exacerbated by the fact that over 12,000 of those Soviet tanks were T-34s; which were significantly better than any widely produced German model of that year. In 1944, however, Germany produced 17,000 tanks (excluding light tanks), to the Soviets’ 21,000 (excluding light tanks). 5,000 German tanks were Panthers or Tigers; which were qualitatively superior to most Soviet tanks.
For the Axis to have had a realistic chance of victory, the following would have needed to occur:
The U.S. would have needed to stay neutral.
American military production would have needed to remain neutral (no Lend-Lease)
Hitler would have needed to wait until 1944 to invade the Soviet Union.
Germany would have had to obtained a 1944 military production level at or near historical levels without prior access to Soviet POW manpower or resources obtained from conquests of Soviet territory
The German military would have needed 12 million people (historic 1944 level) rather than 8 million people (historic 1941 level).