Don't know if it's true, but I heard that Boeing choice of 707 was dictated by the fact "7" is a number with "good fortune" properties. They simply incremented the center digit as projects came to reality, apart from 720 whose launching customer (United, I think) wanted a number higher than 707 to indicate it was a new and more performant aircraft. The second block (B747-
228, for example) indicates the sub-type (first digit) and customer (second and third digit). In the above example it's a -200 B747 whose customer is Air France (28). Letters can be added to show special capabilities, or even replace the sub-type number : B747SP (Special Performance) for example.
Airbus story for 3xx comes from their first airliner, the A300, designed to carry... 300 passengers. Again the center digit was incremented the same way Boeing did. Sub-types numbers or letters are used too, but I don't really know the way Airbus uses them. Maybe someone else could help ?
