Courtesy of Russian Air Force contact;
During World War II, the Sovjet Union needed immediate airlift capacity which it found in the form of an already proven transport aircraft of the time, the US American Douglas DC-3 / C-47 Dakota. From 1943 onwards, no less than 700 US-built C-47 were delivered to the USSR under the lend-lease scheme.
Parallely, the Sovjet Union had acquired the patent to built a derivative of the C-47 under license. Boris Lisunov, who had stayed with the Douglas Aircraft Company for over a year, modified the DC-3 into its Sovjet counterpart. It was initially named PS-84 (PS simply standing for pasashirskii samolet i.e. passenger aircraft) and only renamed the Li-2 in 1942. It resembles the DC-3 / C-47 externally to a high degree. The use of different engines and other details such as additional windows behind the cockpit and a right hand side passenger door (also used on pre-war DC-3A in the US!) distinguish the "Russian DC-3" from its Californian parent.
Apparently as original US design had the RH side pax door and these were the plans passed to the Russians...