Navigator Nolan Herndon said in an interview with The (Columbia, S.C.) State newspaper in 2006 that the pilot of his plane, the one that landed in the USSR, Edward “Ski” York, and the co-pilot, Bob Emmens, were under top secret orders that took them specifically into the Soviet Union after the run over Tokyo.
Herndon, who died the following year, cited several unusual occurrences that pointed to a secret mission:
A 16th plane (his) was added to the mission at the last minute;
York and Emmens weren’t supposed to go on the mission. Doolittle added them.
Herndon and the rest of the fliers assigned to Plane No. 8 had trained with other crews, on other planes.
The carburetors of Plane No. 8 carburetors were altered to burn more fuel than the other planes, even though Doolittle had ordered mechanics not to tinker with the planes’ engines. The change caused them to run out of fuel earlier.
At the time, the United States was trying to persuade the Soviets to allow Americans to use airfields around Vladivostok to bomb Japan.
York and Emmens went on to high-level positions in military intelligence.
Also, Herndon once asked Gen. Jimmy Doolittle about the flight to the Soviet Union and received a cryptic answer.
The general said, “I’ll tell you one thing, Herndon: I didn’t send you there.”
The Soviets thought they were spies, and likely were correct.
“At the time, the United States was trying to persuade the Soviets to allow Americans to use airfields around Vladivostok to bomb Japan.”
They don’t so much sound like spies as they do a sensitive mission to — as you say — persuade the Soviets to make available some airfields. They may have been REGARDED as spies by the Soviets, which was the normal Soviet procedure for any foreigner on its soil. Remember, Russians are the most insecure and paranoid people on the planet.