Which Boeing 737 Next Generation variant has the longest fuselage?

The Boeing 737-900ER has the longest fuselage among the Boeing 737 Next Generation variants.

The 737-900ER is a stretched development of the 737-900. Boeing added a pair of exit doors, a strengthened flat rear pressure bulkhead, and provisions for additional fuel to increase passenger capacity and range. Its fuselage length is about 42.1 metres, making it longer than the 737-600, 737-700, and 737-800.

The aircraft entered service with Lion Air in 2007. It was designed to compete for routes requiring more seats than the 737-800 while retaining the operating commonality of the 737 family. The 737-900ER later formed the basis for the Boeing 737 MAX 9, although the MAX 9 is a separate generation with new engines and other major changes.

The “ER” suffix means “Extended Range.” It does not mean that the aircraft is the longest-range 737 in every configuration; this question concerns fuselage length within the Next Generation family.