The Boeing 737-400 is the stretched version of the second-generation Boeing 737. The major change of the 737-400 over the smaller 300 is a 3,05 meter fuselage stretch, the stretch increases maximum passenger seating to 188. To cope with the increased weights, more powerful CFM56s are fitted. Other changes are minor, such as a tail bumper fitted to protect against over rotation at takeoff, something that could have become a problem due to the increased fuselage length.
All three second generation 737 models share extensive systems and structure commonality, and a common aircrew type rating.