I've never understood the numbering scheme for jetliners. Why do the Boeing series planes use the 7-7 pattern? Why did the McDonnell-Douglas planes have the DC- designation? Somewhere in engineering or marketing at each corporation there is some sort of logical answer, I'm sure.
Each manufacturer used there own naming/numbering system--sometimes an internal use number and also a different public use number or name. Really no different than automobile, bus, truck, or even locomotive practice.
Douglas started out with Douglas Commercial 1 (DC-1) and continued consecutive numbers up to the DC-10, with subtypes such as DC-9-10 or DC-9-50, etc. When they merged with McDonnell, they modified it to MD-11, etc.
Boeing early on had models such as the Boeing 247, the 377 (also known as the Stratocruiser), the 314 flying boat. When the first commercial Boeing jetliner came out, it was internally the 367-80, but was called publically the 707. Later derivatives were the 720, the 707-320, then the 727, 737, 747, 757, 767, 777, and now the 787, with many subs of each type.
Lockheed had 749 Constellations, and derivatives, L-188 Electra's, L-1011-385's, L-1011-500's etc.
Each of these aircraft when built for the military had a separate military designation such as VC135 (707's built for Air Force One usage).
Convair had CV240, 340, 440, 580, 600, 880, and 990.
Martin had 202's, 404's.