Boeing 737-400 Alaska Airlines

737 
The Boeing 737 is a short to medium range, single aisle, narrow body jet airliner. Originally developed as a shorter, lower-cost twin-engine airliner derived from Boeing's 707 and 727, the 737 has nine variants with the -600, -700, -800 and -900 currently in production.

First envisioned in 1964, the 737 first flew in 1967,[1] and entered airline service in February 1968.[1][5] The 737 is Boeing's only single-aisle, narrow-body airliner currently in production, sometimes serving markets previously filled by 707, 727, 757, DC-9 and MD-80/90 airliners.

The 737 has been continuously manufactured by Boeing since 1967 with over 6,000 aircraft delivered and 2,000 orders yet to be fulfilled as of May 2009.[6] The 737 series is the most-ordered and most-produced jet airliner in history as of April 2009.[1] There are over 1,250 737s airborne at any given time, with one departing or landing somewhere every five seconds on average.[7]