The Boeing 737 has been around since the 1960s. There are many models of the plane, but the 737 Max, which debuted in 2017, has encountered one problem after another over the past five years. From two fatal Max 8 crashes in 2018 and 2019 to the recent incident of a door jam falling off a plane during takeoff from Portland, Oregon. CNBC explores how the 737 Max crisis unfolded and the future of Boeing’s best-selling plane.