Question: Where can I find aircraft production statistics for the years just before and during World War II?
Answer: Proquest Statistical Insight (available only to Stanford users) is a good database to start your hunt for statistics. Using Statistical Insight, I used the search terms: (aircraft or airplane) and production.
One of the first five publications to appear in the results list was "Aerospace Facts and Figures". Proquest's digital holdings only went back to the early 1980's so I searched the title "Aerospace Facts and Figures" in Searchworks. I found we have print volumes in Green Stacks back to 1945. In addition if you use the "browse around" function in Searchworks, you will see that Aviation Week and Space Technology is nearby in the stacks, and this journal goes back to 1916, and they published production figures sporadically -- Since these old issues are not indexed, you just have to browse through the issues -- they probably pick one issue a month or year to report summary statistics.