I remember reading a book about the Doolittle raid when at school, I would estimate that I read it about 32 years ago, but possibly a little longer.
Despite the additional fuel tanks fitted, more extra fuel was carried in 5 gallon cans. After takeoff, as the tank contents decreased, these were emptied into the tank(s) inside the fuselage. The additional range was intended to allow the aircraft to make it further into China as the trip was expected to be very marginal otherwise.
The writer of the book (his name escapes me) clearly indicated his concern over the process as it was impossible to avoid some fuel spillage and hence risk fire or explosion.