You are right, checking numbers there seems to have been a slight improvement after 1930-s:
http://www.aerodyn.org/HighLift/ld-tables.html
14,7 for DC-3, and in the range 16...19 now. Wonder where DC-4 and Boeing 307 fall?
But at least after 1960-s... B707-320 has 19,4 and everything else has lower L/D. A320 has 17.