A Northrop Alpha on display at the Smithsonian Air and Space Museum in Washington, D.C.

The Northrop Alpha was a monoplane that seated seven inside the cabin, while the pilot sat in the open air up front.

John Northrop had already designed the Lockheed Vega, one of the fastest single-passenger planes of the day. (Amelia Earhart owned one, and flew it solo across the Atlantic Ocean.) But Northrop left Lockheed and in 1927 founded a new aircraft company, Avion Corporation. In 1929, the United Aircraft and Transport Corporation bought out Northrop; Avion became a subsidiary of United, and was renamed Northrop Aircraft Corporation.

Northrop built his new plane based on two new innovations. The first was the fillet -- a concave piee of metal welded into place where two pipes met. This radically increased the strength of the weld, and allowed for stronger airframes and wings. The fillet also meant far less drag, enabling the plane to fly longer, higher, and faster.

The other innovation was the stressed skin. Most early planes used a rigid frame, over which a non-load-bearing skin was tacked. But this meant lots of internal braces, limiting interior space and making the plane heavy. The alternative was the monocoque, in which the skin itself was thick and heavy and bore the load. But monocoques were limited in size, and tended to collapse if they got to big.

The stressed skin was radically different. In some places, bracing was used to prevent compression. But in others, wires were used to provide tension -- which also prevented compression. Furthermore, the skin of the aircraft was like a monocoque's. But the skin was formed so that it was in tension with these elements. Where braces were used, it was formed to pull on the brace. Where tension wires were used, it was formed to push inward. All these elements made a vastly stronger airframe -- but one that was lighter than anything seen before.

Northrop's stressed skin was revolutionary, and later led to the development of the DC-3.

The Alpha was Northrop's first plane to incorporate both elements. It was very fast, and first flew in on April 20, 1931. Although only 17 were built, they stayed in service for more than a decade.