No, the Boeing 737—regardless of the generation (Original, Classic, NG, or MAX)—is a twin-engine aircraft. It features two turbofan engines, one mounted under each wing. This design is a hallmark of "narrow-body" short-to-medium haul aircraft, optimized for fuel efficiency and lower maintenance costs compared to the three-engine "tri-jets" of the past, like the Boeing 727 or the McDonnell Douglas DC-10. In 2026, the 737 MAX series uses the LEAP-1B engines, which are significantly larger than previous models, requiring the "flat-bottom" nacelle shape to maintain ground clearance. If you see a plane that looks like a 737 but has a third engine in the tail, you are likely looking at a historical 727 or a smaller regional jet like the Yak-42. The twin-engine configuration is so reliable in 2026 that it is used for almost all modern commercial flights, including the "ETOPS" long-haul journeys across the Atlantic.