The United States benefitted economically from WWII in at least two major ways.
First, a number of new technologies were invented or advanced. For example, the creation of long-range bombers really helped towards the creation of larger and longer-range passenger airplanes. The invention of radar helped make it safer for these airplanes to fly from place to place.
Second, many of the other countries that had been economically powerful were devastated by the war. This meant that the US was really the only major economic power whose land and infrastructure was untouched by the war. Therefore, the US had a huge advantage over the rest of the world since it did not have to rebuild after the war -- instead, it could sell stuff to all the people who did have to rebuild.
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