 Though initially fitted with turbojet engines, the dominant engine for the Boeing 707 family was the Pratt & Whitney JT3D, a turbofan variant of the JT3C with lower fuel consumption and higher thrust.JT3D-engined 707s and 720s were denoted with a "B" suffix.While many 707-120Bs and -720Bs were conversions of existing JT3C-powered machines, 707-320Bs were available only as newly built aircraft, as they had a stronger structure to support a maximum takeoff weight increased by , along with modifications to the wing.