That goes back to Boeing's very first commercial jet, the 707 (pictured), first flown in 1957. Theoretically, that could have just been called the 700, but Boeing stuck a "7" on the end instead.

The Boeing 707's success in the U.S. led to Boeing receiving large orders for the aircraft from overseas. The first non-U.S. carrier to operate the aircraft was the Australian carrier Qantas.

At 12:30pm PST, N708PA took off on the Boeing 707’s first flight. However, poor weather soon set in, and they landed just 7 minutes later at Boeing Field, Seattle.