Boeing 707 Douglas competition, Boeing soon realized this would not provide a viable payload, so it widened the fuselage to to allow five-abreast seating and use of the KC-135's tooling. Douglas Aircraft had launched its DC-8 with a fuselage width of . The airlines liked the extra space and six-abreast seating, so Boeing increased the 707's width again to compete, this time to . The first flight of the first-production 707-120 took place on December 20, 1957, and FAA certification followed on September 18, 1958. Both test pilots Joseph John "Tym" Tymczyszyn and James R. Gannett were awarded the first Iven C.
Boeing 707 tail fin on all 707 variants, as well as add a ventral fin, which was retrofitted on earlier −120 and −220 aircraft. These modifications also aided in the mitigation of Dutch roll by providing more stability in yaw. 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,