>> Then Boeing started to build Jets. The prototype of the later 707 was called Boeing 367-80 (so you have the 7 at the end again). But when the final model was about to come out, they decided to give it a new, catchier name and opened the 700 row and because there always has to be a 7 at the end, the first possible designation was 707.
Well... sort of.
At the time that the 367-80 debut, Boeing realized that they had diversified into so many new products that they needed a better naming system. Boeing decided to break-up nomenclature into the following categories:
400-499 = military products
500-599 = missile products
600-699 = turbine products
700-799 = commercial products
800-899 = avionics products
That's just off the top of my head, so they may be a little off. I do know that the 700-series is, for sure, commercial products
Anywho, as Boeing was trying to work the 367-80 prototype into a viable, marketable product, they went through 7 concepts. By sheer coinscidence, the final product was the 707. Even after it went to market, Boeing didn't catch-on: their next product (13 concepts later) was the 720. By then, Boeing realized the tremendous marketing advantage of a number which is universal in all languages. A 707 is a 707 in Spanish, French, German, Chinese, whatever.
All subsequent Boeing products have been 7x7