from the following website:
http://www.afpc.randolph.af.mil/medical/PhysicianUtilization/MC Orientation Guide.doc
III. PROFESSIONAL DEVELOPMENT
f. Residency Training Programs/Graduate Medical Education (GME)
The Air Force sponsors residency training programs in a broad range of medical specialties. Some programs train at MTFs, others are sponsored in civilian facilities, and still others are combinations of joint services and civilian programs. The number and type of residencies made available depends on manning projections for each specialty field, based on the number of physicians currently in practice adjusted for estimates of the number entering/leaving the Air Force over time. Residency training generally obligates a physician to an active duty service commitment (ADSC) based on the duration of training.
Physicians are not limited to training in a single specialty, and it is not unusual for physicians to seek fellowship training or second residencies. Generally, a term of service of at least five years is required before you will be allowed to apply for additional residency training. Fellowship training is not similarly restricted, though certainly time spent in the field makes a candidate more competitive for fellowship training slots. Make sure you are aware of the training obligation and pay implications of any planned residency training before you apply.
A call for GME applications goes out each summer, with selections made late November/early December for training starts in the following year.