By 1987, the 737 was the most-ordered plane in commercial history. In January 1991, 2,887 737s were on order and Models 737-300, -400 and -500 were in production. By 1993, customers had ordered 3,100 737s, and the company was developing the Next-Generation 737s -- the -600, -700, -800 and -900. The Next-Generation 737 models build on the strengths that made the 737 the world's most successful commercial airliner, while incorporating improvements designed for the 21st century.