Originally Posted by
Brian Abraham
As an aside the original Boeing 717 was a 1960's derivative of the 707 that became the KC-135 which is what you may be alluding to.
A little pedantic I know, but I don't think it's correct that the original 717 was a
derivative of the 707 - the two were different airframes but were developed concurrently, the former as a military tanker transport and the latter with a slightly wider body for civil passenger transport. Remember also the original designation of 367-80 (often known as the Dash 80) was a feint.
As for Airbus, the second Airbus model, the A310, was originally designated the A300-B10, so I suppose the series continued from that point to the A320 and so on.