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.