This is the Boeing 221A Monomail aircraft. This plane carried 8 passengers and mail for the San Francisco - Chicago  U. S. Mail Route.

It appears the U.S Mail Service pushed the aircraft/engine industy to design lighter and faster aircraft.

The Monomail's overall design was too advanced for the engines and propellers until the Boeing 247 with new engines and variable pitch propellers came along. Credits: All information is from Boeing's History on their web site. Flying and history is the best.

Best, Mike

Thanks for Looking.

The Monomail design was ahead of its time due to the all metal, round and sleek fuselage, retractable landing gear, anti-drag cowl, and the all metal, low cantilever wing. Unfortunately it required a low-pitch propeller for takeoff and climb and a high-pitch propeller for cruise. Then came P & W Twin Wasps and Hamilton Standards variable-pitch propellers and the rest is history.
The B-9 bomber based its design on the Monomail and could fly at 186 mph, with payload and a crew of five. This was 5 mph faster than any fighter plane of the day. Future fighter and cargo aircraft manufactures took notice. I think Douglas Aircraft engineers took notice, the great DC-3 still flies. As they say, " the only replacement for a DC-3 is another DC-3".

Best, Mike