Two months after the first commercial flight of the Boeing 707 with Pan Am on October 26, 1958, National Airlines became the first U.S. carrier to operate domestic flights using the Boeing 707 ...

Western also embraced the Jet Age, adding the Boeing 707 jetliners to its fleet in 1960. (Pan Am had introduced them in late 1958, and the first one to land at LAX did so in January 1959.) ...

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.