The 737-900ER, formerly known as the 737-900X, increases the capability of the Next-Generation 737 airplane family. The 737-900ER is the same size as the 737-900, but, with the addition of a pair of exit doors and a flat rear pressure bulkhead, can carry 26 additional passengers, raising the maximum capacity from 189 to 215 in a single-class layout. Aerodynamic and structural design changes, including strengthened wings, a two-position tailskid, enhancements to the leading and trailing edge flap systems, and optional Blended Winglets and auxiliary fuel tanks, allow the 737-900ER to accommodate higher takeoff weights and increase its range to 3,200 nautical miles (5,900 km), making it comparable to the 737-800's range.