It's not a great comparison because Germany could not hit the US mainland. Even if there had been a single giant everything factory it wouldn't have mattered.
Then take the Sten submachine gun, designed so that every little machine shop in Britain could produce one.
IIRC the US did plan in case either Japan or Germany somehow became capable of bombing the states. Some factories had their roofs painted to make them blend in with the surroundings, others were built out of reinforced concrete to make them bomb proof
https://airmail.news/arts-intel/highlights/masters-of-disgui...