In the Air Force, promotion is still guaranteed all the way to the rank of lieutenant colonel. Promotion to major occurs six years to the date after graduating from medical school. Promotion to lieutenant colonel is automatic at the 12 year anniversary, although some people will be promoted below the zone if they complete Air Command and Staff College as well as perform some supervisory roles. Promotion to full colonel typically occurs at the 18 year anniversary, but is not automatic. It requires completing Air War College and a number of local and national administrative roles such as chief medical officer at a small hospital, consultant to the Surgeon General, flight commander, and so on. Becoming a general in the Air Force requires completing a flight surgery tour – and preferably residency in aerospace medicine. If you look at the
bios for all of the medical corps generals in the Air Force, you'll notice that every single one of them has been a flight surgeon. The Air Force is the only branch that openly discriminates against other specialties and will not allow anyone else to become a flag officer.