As far as Boeing, they had models with 7 in it, and when they came out with a new model, the Dash 80, the marketing guys thought 707 would work better than the older way (Model 367-80 I think, correct me if I'm wrong!). Airbus chose A300 because it was supposed to stand for "A"irbus "300" passenger. It turned out to be 250 passenger, but the name stuck. All the names after that are obvious. McDonnell Douglas is obvious too, DC (Douglas Commercial) Model 8, 9, 10, or McDonnel Douglas, Model 88, 90, 11. Hope that helps.