From this spreadsheet, it shows that the airstream clipper (1936) was manufactured when the aluminum production was doubled from the previous year. Also, the low unit value of aluminum in the late 1930's and 1940's may have contributed to the choice in material of the airstream clipper.
"Aluminum Statistics." USGS. N.p., 2005. Web. 5 Nov 2012. <http://minerals.usgs.gov/ds/2005/140/aluminum.pdf>.