Conversation so far:
Did anything specific happen in the later 1930s for Duke Ellington?
Billy Strayhorn became a vital member of the Duke Ellington organization and the 1930s ended with a very successful European tour just as World War II loomed in Europe.

Question: What kind of recordings did he do?
Self-contained rewrite: What kind of recordings did Duke Ellington do during the later 1930s?