MiTasol
1st Lieutenant
Not a classic WW2 aircraft but there was a large "baggage" space accessible from the starboard side behind the cockpit on the Seversky P-35. The aerofiles site shows one P-35 fitted out with a buddy seat..
Even the ones without a buddy seat were used to carry passengers at least once
From Bill Bartsch's book Doomed at the start, page 238