The Boeing 737 is a narrow-body aircraft produced by Boeing at its Everett and Renton factories in Washington. Developed to supplement the Boeing 727 on short and thin routes, the twinjet retains the 707 fuselage width and six abreast seating but with two underwing turbofans instead of four. Envisioned in 1964, the initial 737-100 made its first flight in April 1967 and entered service in February 1968 with Lufthansa. The lengthened 737-200 entered service in April 1968, and evolved through four generations, offering several variants for 85 to 215 passengers.