My Father was a medic in WW II. he was a Tech Sargent(just above Buck Sargent). Most of the officers who performed Operations on the Wounded in the field and also performed other functions associated with their status as an MD were Lieutenants.
It is a shame that we are not able to get the qualified MD personnel in the Military without giving them high ranking levels. A Battalion Commander has responsibility for 1000 men and women in combat.
In WW2 you could simply draft an intern or a resident into the army. No need to worry about rank as long as he was an officer. In a volunteer force you need to attract doctors. I knew a guy that enliste in the USAF so that he’d get medical school paid for. Instant O-2. He could barely wear the uniform correctly. On graduation, automatic promotion to captain. I imagine that major is pretty much automatic to induce a re-up.