The Boeing 737 is a short to medium range, single aisle, narrow body jet airliner. With over 7,000 ordered and over 5,000 delivered, it is the most ordered and produced commercial passenger jet of all time. Born out of Boeings need to introduce a competitor in the short-range, small capacity jetliner market, it has been continuously manufactured since 1967. It is now so widely used that at any given time, there are over 1,250 airborne worldwide. On average, a 737 takes off or lands every five seconds somewhere around the world.
Boeing was far behind its competitors when the 737 was launched, as rival aircraft were already into flight certification. To speed up the development time, Boeing reused 60 percent of the structure and systems of the existing 707, most notably the fuselage cross section. The 737-800 was a stretched version of the 737-700. It filled the gap left by Boeings discontinuation of the McDonnell Douglas MD-80 and MD-90 after the two companys merger.
Major United States airline Delta Air Lines has an all-Boeing fleet. It was one of the last major airlines to operate the original Boeing 737-200 models until the last aircraft retired in 2006. Its many Boeing 727s were completely replaced with Boeing 737-800s in 2003. Deltas Boeing 737-800 airplanes have an empty area in the rear cabin where seats are normally located. Not placing seats in this area results in a reduction of capacity to 150 seats, reducing the number of Federal Aviation Administration mandated flight attendants to three, as one flight attendant is required for every fifty passenger seats. This seat reduction was done after 9/11 to reduce costs, however, Delta has backtracked on this decision, and will increase the capacity of the 737-800 fleet to 160 passengers by using slimline seats.