Two primary versions of the aircraft were built. The original 720 with Pratt and Whitney JT3C, turbojet engines entered service in 1960, while the improved 720B with Pratt and Whitney JT3D turbofans entered service in 1961. Some 720s were later converted to 720B specification. Although only 154 were built, the Boeing 720/720B was still profitable due to the low research and development costs of it being a slightly modified version of the 707-120. It was later replaced by the Boeing 727.