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Sorry if this has been covered, but what is the origin of the "7_7" names?
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The Boeing historian Mike Lombardi replied to my Email about this very subject a few years ago. He told me that the marketing department was responsible for the 7x7 designators with the birth of the "707" as they thought the illiteration of the format was very favorable, in addition to sevens being 'lucky' and -700 series for commercial a/c concepts. The mathematics idea about the 707s wing was nice, but someone showed how it didn't truly work out and it's an urban legend. (Can't find it right now.) However, here's an excerpt about this topic-
Whatever happened to the 717?
by Ed Brown
Fortune magazine; 2/2/1998
Boeing announced in early January that it is rechristening the McDonnell Douglas MD-95 as the Boeing 717-200. But why wasn't there already a 717 in the skies? There's the 707, the 727, the 737.... The gap hints of an Edsel-like disaster in the company's past. But it's not that. The first 717 had its day--as a military cargo and airborne-refueling plane called the KC-135.
In the early days, says Boeing historian Mike Lombardi, the company numbered its products sequentially, starting with 1. Boeing kept this up until the 1950s, when it assigned a set of numbers to each of its product lines--600 for missiles, 700 for commercial jets, etc. Every significant initiative within a given series, even if it was just a sketch, got a number, which explains why Boeing's first commercial airliner, unveiled in 1954, was not the 700 but the 707. Then the marketers decided they liked the palindromic ring of "707" so much that every 700-series jet name would end with 7.
The prototype that produced the 707 also spawned a military jet, which was marketed as the 717. Once the U.S. Air Force renamed the plane according to its own classification system, however, everyone--including the folks at Boeing--started calling it the KC-135. Boeing stopped making the plane in 1965, but about 550 of them are still in use by the Air Force. "