The Boeing 737 has many different models. Starting with the 737-100, it was a small, short-haul aircraft. As it progressed, the 737-400 became longer, with more powerful engine options, and now, the 737-900 series is on its way, after its predecessors, the -100, -200, -300, -400, -500, -600, -700, and -800, it is the most experienced, and once of the most up-to-date, aircraft of our time. There is great difference is both the design and length of these aircraft models as some later modles can be used for medium-haul routes.