According to "Legend and Legacy" by Robert J. Serling it is more or less a
PR-gag, why Boeing uses the 7x7-designations.
"Each type of Boeing commercial jetliner is distinguished by a middle numeral between two 7s. The 707 should really have been the 700, the first of a new type of Boeing product, following earlier commercial aircraft or military aircraft whose numbers began with 2, 3, or 4 (Numbers 5 and 6 were used by missiles and pilotless aircraft.) The only explanation - a feeble one - for why it became the 707 rather than the 700 was that the Public Relations Department thought it would be a good idea."