I recall from the book " Thirty Seconds over Tokio " by or about one of the pilots, Ted Lawson, that the Mitchells had to take off prematurely owing to an unexpected encounter with a Japanese merchant ship.
The vessel was quickly disposed of by the Hornet's escorting destroyers but it was feared that the ship might have had time to transmit a report of sighting the US ships.
This early take off, if I remember correctly, resulted in most of the planes failing to reach their intended destination, Chunking. One of them wound up landing on Russian territory and the crew were interned. No doubt the Soviets were delighted to have the opportunity to examine the plane.
Lawson's B-25, nicknamed The Ruptured Duck, crashed in the surf off the Chinese coast when the engines cut while lining up for an emergency landing on a beach.
Ted Lawson suffered leg injuries which resulted in his losing them, and the bomb aimer, named Cleverley, died as a result of his wounds before the crew could reach safety.
It is now a long time since I read "Thirty Seconds over Tokio" but I think I have the above right.