The "Avaition Engineering" column in Aviation for April, 1940, included two short news clips, with photos:
One of the important details in the development of the Boeing Stratoliners that has gone by practically unnoticed is the complete working wind tunnel model.
The 307 research model is just one-tenth the size of the actual airplane, which has a wing span of 107' 8", and a length of 74' 4". Built in the Model Department of the Boeing Wood Shop, the miniature Stratoliner has wings of laminated mahogany, and tail and control surfaces of maple. The four engine nacelles, each housing a compact 5 hp electric motor, are machined out of cast dural and the motors drive 18.8" three-bladed propellers, at speeds up to 20,000 rpm.
Inside the fuselage of the model are two more small electcic motors which operate the rudder, elevators, ailerons and wing flaps. During a test run in the wind tunnel, researchers can move any of the surfaces by remote control. Each change in position of the control surfaces, including the deflection caused by the force of the wind is automatically and accurately indicated on the central instrument board along with other pertinent test data.
The Flying Cloud, the new name given to the Boeing Stratoliners by Pan American, is about to go into regular service. But inside the ship is a cabin pressurizing device which solved a problem that caused many an engineer in the industry to use descriptive words not yet described in Webster's best unabridged edition.
In each of the two inboard nacelles is located one of the superchargers being driven off of the engine. On the leading edge of the wing, one on each side of the fuselage is located a circular hole approximately 5" in diameter where the air entering the intake side of the supercharger is supplied. When the ship is flying at an altitude that does not require supercharging a special vent bypasses the air from the intake pipe back to the outside again. After being supercharged the air passes through conduits and is distributed to the cabin through small grills located just above the floor level along the sides. The air of course is heated and contains the correct amount of oxygen.
All parts such as doors and windows have been designed to hold at least a pressure of 2½ psi inside the cabin. The passenger door, incidentally, opens inward rather than outward to facilitate maintaining the pressure.