The initial titles state that the film starts in November 1941. When Johnny Williams receives a telegram in Mandalay shortly after, it is dated 8-6-41, i.e August 16, 1941.
The Japanese infantrymen are wearing German helmets.
Luchow (now Luzhou) to Mandalay is a 750 mile flight, impossible without refueling in the type of plane commandeered by Montgomery.
When George Montgomery pours water on Chandu to wake him up, you can see the brown make up washing down the chest of actor Robert Blake.
The vehicles are all left-hand drive. In the 1940s Burma's traffic drove on the left-hand side of the road and the vehicles were all right-hand drive. (This changed in 1970.)