The massive F-105 Thunderchief is famed for its role in Rolling Thunder during the Vietnam war. Its early service, however, was plagued by technical issues. The first production model, the F-105B, was quite unsatisfactory, and only 71 were built. Lessons learned from that early model, coupled with improved avionics, were implemented in the main production model, the F-105D.
Here’s a Republic Aviation marketing film showing some of the features of the early F-105B. The easiest visual cue to distinguish the B from the D is the shorter nose of the B model, with the gun port of the M61 being much closer to the tip of the nose.
Republic F-105 Thunderchief Avionics: "A Link to the Future"
Very old school