Boeing 720
The 720B was a different type of 720. It had JT3D turbofan engines. These engines made  of thrust each.  The JT3D engines burned less fuel and could make more thrust. The first 720B took off on October 6, 1960. The first airline to use it was American Airlines in March 1961.  89 720Bs were made. American Airlines also changed some of their normal 720s to 720Bs.