Hello! Mass production is an intensive form of work that requires a lot of capital and energy, since it uses a high proportion of machinery and workers who must follow rigorous instructions. It is automated, so it requires steps proposed for proper operation.
The greatest advantage of United States in world war II was its ability to rapidly transition from peace to war and mass produce weapons and war equipment at a colossal scale. This was so effective that the USA was able to make up for time lost, and the nation was able to effectively train the necessary forces and then exert a massive material superiority.
America was able to build up an air force that came to dominate the skies, and with this air superiority, it was all but over for the enemy, the Axis forces.