Tac: I am also sure that there was a prototype B-50 bomber, too. That is how those numbers were assigned.
I was trying to make a little light humor about an issue that really doesn't matter to anyone, more than just curiousity. I suspect that any number for a plane that does not represent an existing, or past model that was actually put into production and used in our military air forces will be subject to being used on some future aircraft.
The first bomber that had a number that I ever heard or read about was the B-24. No one ever asked what happened to the first 23 models! I am sure that the companies that produced the bombers put such numbers on those earlier designs. They didn't find buyers for them, tho', so they designed new planes to see if the next ones would sell. Before and during WWII, fighter planes were designated with a " P " in front of their model number. Bombers got a " B ". When jets came out after WWII( actually they were being developed throughout the war) they were given the "F" designation for fighter. The F-80 " Saberjet" is the first American Jet fighter that I recall ever seeing, or reading about. I know there were earlier models.
I also know that they make 5Fg powder, but Have never seen any for sale, and haven't a clue why anyone would buy it. Since I made my first Bomb with powder scraped from unexploded firecrackers carefully recovered on the Morning of the Fifth of July over several years, together with powder from unfired .30-06 blank rifle cartridges gathered from the parade routes, I am well aware of the " dust " that is 7Fg powder. I don't have any use for it either, unless I wanted to make a fuse with the stuff.