To repeat the poster above you, correlation doesn't equal causation.
Secondly, the US's biggest contribution to the war was manufacturing capacity. Period. Sure, some of the things built in America were designed elsewhere, but the US could outproduce everyone by an order of magnitude thanks to abundant natural resources, a large population, and a heavily industrialized manufacturing base. Not being bombed isn't really a factor in that equation.
Also, America produced its share of extraordinary weapons during (and before) the war -- the Thompson submachine gun, the Jeep, the 50 cal machine gun. American submarines were first rate along with aircraft like the P-51. American trucks were the backbone of both the Western and Eastern Allied supply lines.
It's pretty disengenuous to summarize Americas contribution to the war as you have above
While I don't necessarily agree with the statement, it is interesting to see how many things the Brits invented post 1900, but did not have the wherewithal to fully commercially exploit. For more on this, I recommend visiting the science museum next time anyone here is in London.
Of course we had more stable manufacturing post world war 1, where we took only 10% of the losses of the other developed nations, and 2% of the worlds population was wiped out. It's not surprising that a country who lost only 0.13% of its people was able to move faster and build more stuff than a country who lost 13% of its total population.