The Boeing Company had a history of naming its' aircraft with numbers going back to to 1917.
Post WWII they were up to the 400 series (B-52 bomber was model 464). 500 series was reserved for powerplants while 600 series items were missiles. Next in line was 700 and while originally named internally the 367-80 or Dash 80 the aircraft was officially christened the 707.
Some say there are ! metaphysical - illuminati - masonic ! (cue strange music) reasons for the relationship between Boeing and the number 7.
Anyone care to explain the Airbus relationship to the trinity ?