Gasoline mixture had been previously adjusted for maximum “mileage”; however, while on the carrier, the mixture was changed back to the “book” settings. Just in time, the error was noted and the mixture reset. (!)
Amazing, the pilot training manual for the B-25 is online.
There could have been another inaccessible adjustment but it looks like the mixture is adjustable from the cockpit. The pilot and crew would probably have been keeping a close eye on it as they progressed towards Japan to be sure they weren’t consuming too much fuel.