May be off topic because Boeings are not homebuilt.

I've heard all Boeing aircraft had a 7 in their model #, 707, 747,
PT-17, etc. except the B-52 and then even 5+2=7. Is this true ot not. I
know the KC135 doesn't have a 7 but it is a 707. Would that make the 7
in model #'s true?

Saw a special tonight about the B-52 and it made me start wondering again.
L.D.