Boeing 307 Stratoliner
In 1935 Boeing designed an airliner with four engines. It was based on the Boeing B-17 Flying Fortress. This airliner was called the Model 307. It had the same wings, tail, rudder, landing gear and engines as the B-17C. However, the middle of the plane was a circle. Its diameter was . It was designed so that the plane could be pressurized.