Passengers with oxygen masks on Northwest Airlines plane, c.1940.

Air mail being loaded onto a Northwest Airlines plane, c.1940.

Northwest Airlines city ticket office, 100 Sixth Street South, Minneapolis, c.1950.

Northwest Airlines Operations, Holman Field, St. Paul, 1934.

Passengers boarding a Northwest Airlines plane, c.1946. Photographed by Philip C. Dittes.

Interior view of the cabin of a Northwest Airlines Boeing 727 plane, March 11, 1969.

Northwest Airlines stewardess, March 3, 1949. Photographed by Philip C. Dittes.

Northwest Airlines Stratocruiser, c.1955. Photographed by Don Berg Photography.

Northwest Airlines monoplane, 1930. Photographed by Leo J. Kohn.

Northwest Orient Airlines menu, c.1970.