The best-selling Boeing 737 started as a smaller, short-range jet, and 19 737-200s were sold as T-43 Air Force trainers. Models 737-300, -400, and -500 were followed by the Next-Generation 737-600, -700, -800, -900, and the luxurious Boeing Business Jet. The Boeing Next-Generation 737s are the most advanced single-aisle airplanes in the market today. While these new airplanes retain the characteristics that made 737 classics so popular worldwide ~ reliable, simple and economical to operate ~ they underwent dramatic revisions. The 737-700 also is used for Project Wedgetail, an airborne early warning and control system for the Royal Australian Air Force, and for the U.S. Navy's C-40A Navy-Unique Fleet Essential Aircraft (NUFEA)