The problem for the Germans is that in almost all categories US industry is spread out, there are a few critical sites, but for the most part if one place is damaged others can make up for it. Furthermore the distances involved once you get past the shoreline are vast, and the bombers will be subject to air defenses all the way. The only way anything more than nuisance bombing of the USA happens is if the Germans get a base close - one of the French Caribbean islands, Mexico, Newfoundland etc all of which are pretty ASB.