This article provides some interesting information:
1. Enlisted recruits and officers are more likely to come from middle- and upper-class neighborhoods. Can you imagine why? I certainly can.
2. Racial minorities are not "overrepresented" in the military.
3. In general, Democratic-voting states are "underrepresented" in the military while Republican-voting states are "overrepresented".
The charts at the link are fairly interesting.