I've answered several questions related to this topic, but I'll paste the most informative reply below. If you have any further more specific questions do not hesitate to ask.

• During the Draft in America, how did they choose who went to which branch and did what job?

• Regional Differences in US Soldiers' Placements during WWII? Regional Differences in War Coverage?

• How were US soldiers assigned to regiments/divisions during WWII?

• Were Americans who enlisted or were drafted during World War II able to choose the Pacific or European theater?

• Did US troops in WW2 experience continuity of unit integrity as they progressed from basic training to combat?

I assume you're speaking of initial mobilization; turbulent personnel practices that came after in the period of training and the individual replacement system are a whole different animal. Which unit was sent where generally depended upon when it was activated, when it was deemed to have satisfactorily completed training, and the exigencies of the war, rather than the localities of the men within it.

Side note, there was no department of defense during World War 2. The Department of War/ War Department/ War Cabinet is what you are looking for. It was split into the Department of the Air Force/Department of the Army, which was collected under the national military establishment along with the pre-existing Department of the Navy and that collected group was renamed the Department of Defense - this was in 47 iifc.

Edit - the DoD name change was 49, the collection under NME was 47.