In 1991, Boeing started working on an updated group of 737s. They did this so they could have a plane to rival the Airbus A320. Boeing said that the 737 Next Generation (NG) program had begun on November 17, 1991. The -600, -700, -800, and -900 models are all part of the 737 Next Generation program. The 737 was changed so much by the 737NG program that it is almost like a new aircraft. However, some important parts of the plane were kept similar to the older 737s.