During WWII, promotions in the U.S. Army were very rapid. The U.S. Army went from a tiny peacetime force of about 180,000 people in 1939, to 8.26 Million people in 1945, a force forty times that size. Of course, the vast majority of people entering the service were civilians with no military training, and that meant a huge number of vacancies in the officer corps.

If you read the military biographies of the West Point class of, say, 1940, you can see that many of them started as Lieutenants and made Major, Lt. Colonel or Colonel by 1945. There simply was not that much military experience to go around. Even if they were not West Pointers, officers that started their service during 1939-41 saw particularly rapid advancement because they were well positioned for rapid promotion during the army's expansion, which started in the summer of 1940, before the U.S. entered the war.

The U.S. Army also directly appointed people with technical skills to appropriate rank, so medical doctors for example could become relatively high ranking officers upon induction.

Also, there are a lot of special cases. Men appointed to high ranks because of political connections, or just being in the right place with the right skill at the right time.