Numbers are usually a means to an end, but some people find mystical qualities in the numbers themselves. Ever since the ancient Greeks were shaken by the discovery of irrational numbers (and probably before that) people have attached supernatural qualities to certain numbers. You know all quarterbacks want to have an even number, preferably 12, and all running backs will take 32 if they can get it. Some airlines omit row 13 from aircraft, and I am told that in East Asian cultures, the number 4 is associated with death. In Japanese, the word for four is shi, which also means death. Sometimes they say yon instead of shi for four, kind of like we might say darn instead of damn.When I lived in Columbus, Ohio, my Japanese colleagues at Honda often had occasion to take a cab from the airport. Unfortunately, one of the major cab companies chose the phone number 444-4444 and plastered it all over their cabs. On the other hand, East Asians like 8. I once had a phone number full of 8s, and a Chinese friend said I should try to sell it to Asians.
Airplane companies and their marketing consultants have tapped into these superstitions for decades. The basic idea is that a higher number must mean it's a better airplane. In the early days of airliners, a simple sequential numbering system took hold. From the Douglas company came the DC series, with the DC-3 in the 1930s being their first real airliner.To Douglas's credit, they never got fancy with the numbers. By the 1960s they were up to the DC-10, and they continued the same sequence even after they were bought out by McDonnell. They managed to produce the MD-11 and had sketched out an MD-12, but that never came to fruition, and the line came to an end with the buyout of McDonnell Douglas by Boeing in 1997. Perhaps the buyout was facilitated by the knowledge that the next McDonnell Douglas plane would have to be the MD-13. These were the large airliners. McDonnell Douglas did branch off another line from the smaller DC-9, creating a stretched version called the MD-80. That line got as far as MD-95 before the buyout.
Boeing, on the other hand, gave a little more thought to the connotations of their numbers. They reached 377 in the 1940s, and someone must have liked the sound of those lucky 7s, because of course their first jetliner was the 707. (Aside: the 707 was called the 367-80 or "Dash-80" in the prototype stage. There was also a 720 that you've probably never heard of.) They went on to the 727, 737, 747, 757, 767, 777 and 787. They seemingly skipped the 717, but that number had been used internally for a couple of things. When Boeing bought McDonnell Douglas, they slapped the 717 label over the MD-95, but few of those planes ever sold.
Now we get to the real marketing genius. In the late 1950s, Convair, a successful producer of turboprop airliners, wanted to get into the jet business. Their competition was the 707 and the DC-8. They originally called their first jet the Convair 600, which was a continuation of their turboprop line which had reached 580 by then. But some marketing genius must have looked around and seen the 707 and DC-8 sporting "higher" numbers. So they changed it to the Convair 880, with the pretext that 880 feet per second was the planned speed of their new jet. They then went one better with the 990, which of course was supposed to go 990 feet per second. Unfortunately, the 880 and 990 were both beautiful dogs, and they nearly bankrupted the Convair company. But you can still see old Convair turboprops all over the world.
Lockheed had built a sleek and successful piston-driven airliner called the Constellation, with no number used in marketing, but most of the large airplanes they built in the 1950s and 60s were military transports: the C-130, C-141 and C-5. (The military doesn't care about numbers. I make joke here. Numbering of military airplanes is laden with superstition as well.) When Lockheed decided in the mid-60s to build a jetliner, the main competitor was the DC-10. In fact, the design they eventually came up with was almost indistinguishable in appearance from the DC-10. The most obvious difference is that the DC-10's tail engine is not faired onto the fuselage. But this didn't hold back Lockheed's marketers. They said, screw it, we're going to match your bet and raise it: our plane will be the L-1011 (pronounced ten-eleven.) It's like your DC-10, only one number better! The L-1011, like the Convair jets, was a commercial failure. Lockheed lost millions and never built another airliner.
Now, I'm sure that in every one of these cases, the marketing department came up with a cockamamie rationale for why those exact numbers were chosen (the 880 feet per second business is one.) Lockheed probably claimed they had an L-1010 on the drawing board, but it just wasn't up to snuff, so voila, the next plane had to be the L-1011. But I ain't buying it.