How did World War II affect the American economy?

To expand on teachersage's answer, I would like to focus on the impact World War II had on the role of women in the economy. As teachersage noted, with most men away at war, it became the role of women to take up jobs in factories. There is no doubt that women on the home front contributed greatly to making sure that American factories pumped out the materials necessary for the men overseas to win the war. Out of necessity, this marked a departure from the traditional roles of women in society.
Before World War II, women were expected to stay home and take care of the household, or, if they did choose to work, to work in jobs that were traditionally done by women. Before World War II, women were also expected to stop working after marriage, or at least after having children. Upon marriage it became the duty of women to maintain the household and look after the children while the men worked.
Following World War II, many women hoped to remain in the workforce. They had experienced the benefits of earning their own money, and had proven that they could be successful participants in the American workforce. There was a drop in the number of women in the workforce in the years immediately after World War II due to the return of military men from overseas, but women would quickly rejoin the workforce in the decades that would follow. The experience of women in the workforce during World War II led to the eventual understanding that women could not only participate in the workforce, but could do so even after marriage and having children. The economic participation of women during World War II led to revolutionary changes to the role of women in the American economy and in American society in general.

Though World War II was terrible and tragic because of lost lives, destroyed cities, and the Holocaust, it was good for the American economy. Providing materials for the war propelled the United States out of the Great Depression. Starting in 1939, the year the war began in Europe, jobs in the US abounded and the once high unemployment levels dropped virtually overnight. Suddenly, everyone could find a job.
Once the United States got into the war in late 1941, the economic situation only got better. It has been said that supply lines win wars, and the US was a huge supply line, putting its industrial might into churning out a seemingly endless number of planes, tanks, jeeps, uniforms, guns, grenades, and uniforms. With most of the men away at war, women worked around the clock in factories. Because of rationing, there was not much to buy, so people saved their money.
After the war, people quickly converted their savings into material goods, buying cars, clothes, houses, washing machines, and other consumer items in a frenzy of pent-up demand that fueled economic growth. Further, with most of Europe in shambles, the world depended on US factories to supply the goods and materials needed for rebuilding. After the war, the US took its place as the world's economic powerhouse.