The Boeing 737 is easily recognizable due to its distinctive flat-bottomed engines. This design choice emerged from engineering adaptations made over decades as engine technology evolved. The 737, originally designed in the 1960s, featured much smaller engines, allowing it to serve airports with limited infrastructure.
Over time, Boeing needed to adapt to larger and more efficient engines, like the CFM International CFM56 and later the LEAP-1B. This necessitated the flat-bottom modification, as the alternative would involve costly redesigns of the landing gear and other structural elements. Instead, Boeing repositioned and reshaped these larger engines to fit the aircraft's existing design.
Boeing's approach began in earnest in the 1980s when it adopted the CFM56 engine, which had a larger diameter than the original engines. "According to the Airplane Academy, the 737's engine installation is distinctive in the industry and is 'arguably less effective than simply lengthening the landing gear.'" However, the redesign required to fully update the 737 wasn't undertaken, prompting these innovative, if imperfect, adaptations.