The first Boeing 737-800 -- the second out-of-production model in the Next-Generation 737 airplane family -- has been delivered to launch customer Hapag-Lloyd of Germany. The airplane is the first of 16 737-800s ordered by the airline. The -800 models eventually will replace the 737-400s and -500s currently operated by Hapag-Lloyd. With almost 4,000 orders, the 737 is the best-selling commercial jetliner of all time. More than 825 Boeing 737s are in the air at any time, with one taking off every six seconds.