I think one of the biggest benefits to the economy was the fact that so many people were able to be employed. If you look at economic trends you will see that during prosperous times there is low unemployment.
The entry of the United States into World War II required the creation of an economic machine in order to drive the war. Factories had to produce machinery and equipment to be exported to military companies overseas, for example. Technologies were developed to support the war effort, leading to employment of workers to produce the goods.
Women became employed on a large scale in industries other than traditionally female-dominated areas. With so many men enlisted in the war effort overseas, it fell to the women who were left behind to "man" the factories and industries. High employment leads to prosperity.
Another very important and major economic benefit of World War II was that the industrial renewal and large scale employment it caused - both in the military itself and in the industries that supported it - brought us once and for all out of the Great Depression. Without World War II it is unknown whether we could have crawled out of that mess ourselves. We had spent a large amount of money on the New Deal by borrowing, but it hadn't cured us of the economic problems we had.
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.