From The Airport Wiki

The Boeing Model 307 Stratoliner was the first commercial transport aircraft with a pressurized cabin. This feature allowed the plane to cruise at an altitude of 20,000 feet (6,000 m), well above weather disturbances. The Model 307 had capacity for a crew of five and 33 passengers. The cabin was nearly 12 feet across. It was the first plane to include a flight engineer as a crew member.