The Germans were exploring thermals in the 1930s, maybe even in the late 20s. In the 30s Robert Kronfeld came to Britain and made quite long cross-country flights using thermals. and of course, Geoffrey Stephenson's cross-Channel flight in 1938 or 1939 used thermals.
However, Mary is right so far as the early 1930s were concerned. I have a 1932 gliding book which says that the most successful club will be the one which has a horse to pull the gliders back up the hill.