"If you look at past boeing planes, (pre-707) youll notice that they just ended in 7, i.e. 377, 247, etc."
In fact, that only happened by chance. Many of the airliner designs did indeed end in 7---but not all. Boeing also produced the Model 40, Model 80 and Model 314 flying boat, often referred to as the "Clipper". Their first pressurised airliner was the 307 (the Stratoliner), but the original attempt to design this airliner was the Model 300, which was dropped.
Originally, Boeing's model numbers (which were really design numbers) were simply sequential. If a design failed to enter production, its number was not re-used. Military model numbers were mixed in with commercial ones. For instance, the B-17 was the Model 299, the B-29 was the Model 345, the C-97 was the Model 367, the B-47 Stratojet was the Model 450 and the B-52 is the Model 464, just to name some of the better known types.
In the early 1950s, the system (which at that point had reached somewhere around model number 475) changed to one in which separate large blocks of numbers were reserved for different product lines. I believe the 500 and 600 blocks were kept for guided missiles and waterborne craft, although I'm working from a rusty memory here. The 700 block was allocated to civil and military jet-powered transports, and 707 fortuitously popped up as the design number for the first to be produced. Boeing apparently liked the sound of it so much that they changed the numbering system again, and kept the 7x7 pattern for (almost) all the 707's successors. The model numbers became actual names, unrelated to design numbers.
There is one big oddity: the "Boeing 717" designation. The Model 717 is actually the military C-135, KC
-135, etc. When a short-fuselage development of the 707 came along, Boeing first designated it as the 707 series 020. Then they reclassified it as a Boeing 717 (specifically, the 717-020). Neither idea made good sense, since the plane was really a different design from both the 707 and 717 in major ways. With some pressure from United (I now forget why), it oddly became the Boeing 720.
Of course, in recent times, Boeing chose to forget all of this and renamed the former MD
-95 as the 717. (Perhaps they didn't want to use up those precious 7x7 numbers?) It really ought to have been the 787. But then, it doesn't really deserve a Boeing model number at all, as it isn't a Boeing design.
Let's think: if the 717 is really the 787...well, the 7E7 must be the 797!
IMHO, Boeing ought to call it the 787 and get on with it. But (also IMHO) let's hope they drop that ghastly touchy-feely "Dreamliner" nonsense. Uggh. Totally drippy. The sort of name Hoover might give to a vacuum cleaner. Or perhaps it's really fantastic new gel pen? No, no, it's a mattress!
BTW: For anyone interested in the Boeing design/model numbering system---or who would like a truly definitive account of this and much more, I would recommend "Boeing Aircraft Since 1916" by the late (and much-missed) Peter M. Bowers. A little out of date by now, but a fabulous reference nonetheless.
Apologies for the long post. I mean only to be helpful. We aviation enthusiasts are notoriously detail-oriented people, are we not?