I guessed that draftees with lower levels of educational accomplishments got Enlisted ranks, whereas draftees with higher education got ranks of commissioned officers. But on what basis were they assigned differing ranks? Blake was lieutenant colonel, Burns and Winchester were majors, Hawkeye and BJ were captains, Mulcahy was a first lieutenant. Was it based on how many years of experience they had in private practice, or their age, or something else? If so, what was it?
Burns was an ROTC and reached the rank of Major in the Army Reserves.
It's never explained how Winchester achieved the rank of Major, but it's assumed it's based on his tenure and expertise as a doctor. Before being drafted, he was in line to be Chief of Thoracic Surgery at Boston General.
Hawkeye and BJ presumably went through the training program, as they were both very new to the profession, which made them eligible to be Captains without any real authority. All doctors entered the Army as Captains.