The 737 was originally envisioned in 1964. The initial 737-100 made its first flight in April 1967, and entered airline service in February 1968 at Lufthansa.
The 737 series is the best-selling commercial jetliner in history. As of 2006, there were an average of 1,250 Boeing 737s airborne at any given time, with two either departing and/or landing somewhere every five seconds.
According to wikipedia