See the Congressional Research Service (CRS) publication "Military Service Records and Unit Histories: a guide to locating sources." (2018)
Military Service Records are available from National Archives and Records Administration (NARA) via Standard Form 180. If you are a veteran or next-of-kin of a deceased former member of the military, you may now use eVetRecs to request copies of military records online, including DD FORM 214, Report of Separation. Please note that you will have to sign and mail, or fax, the form that is generated by an eVetRecs request within 20 days to complete the process. If you are doing genealogical research in military service records, please see NARA's guide Research in Military Records.
In gathering the necessary information to order records, you may find these other NARA web sites helpful:
WWII Honor List of Dead and Missing Army and Army Air Forces Personnel (You will need to know the home state and county to search this list.)
State Summary of WWII War Casualties for Navy, Marine Corps, and Coast Guard Personnel (You will need to know the home state and county to search this list.)
Ordering military medals and awards (Including duplicates under certain conditions)
Social Security Death Index Interactive Search (Use this site for dates of birth and death and social security numbers, when available.)
Where to write for Vital Records (A useful government guide that offers listings for each state on how to obtain birth, death, marriage, and divorce certificates.)