The 737NG is an upgrade of the 737 Classic (-300/-400/-500) series. Compared to the 737 Classic, it has a redesigned wing with a larger area, a wider wingspan, greater fuel capacity, and higher maximum takeoff weights (MTOW) and longer range. It has CFM International CFM56-7 series engines, a glass cockpit, and upgraded and redesigned interior configurations. The series includes four variants, the -600/-700/-800/-900, seating between 108 and 215 passengers. The 737NG's primary competition is the Airbus A320 family.

In 2004, Boeing offered a Short Field Performance package in response to the needs of Gol Transportes Aéreos, which frequently operates from restricted airports. The enhancements improve takeoff and landing performance. The optional package is available for the 737NG models and standard equipment for the 737-900ER.

In July 2008, Boeing offered Messier-Bugatti-Dowty's new carbon brakes for the Next-Gen 737s, which are intended to replace steel brakes and will reduce the weight of the brake package by 550–700 pounds (250–320 kg) depending on whether standard or high-capacity steel brakes were fitted. A weight reduction of 700 pounds (320 kg) on a 737-800 results in 0.5% reduction in fuel burn. Delta Air Lines received the first Next-Gen 737 model with this brake package, a 737-700, at the end of July 2008.