Western airlines started flying scheduled trans-Siberia in the 1960s. The first was SAS flying DC-8s Copenhagen-Tashkent-Bangkok (remember all the now independent republics like Uzbekistan, where Tashkent is, were just part of the USSR).
And of course they had been flying into the Soviet Union for longer. Finnair were doing so by 1950, British Airways by about 1957 (when Aeroflot started to London as well). IIRC they started with Viscounts, stopping in Warsaw.
Did Lufthansa do Ju52s to Moscow in the 1930s ?