Well, if you think about it, The Mitchells were launced way far in advanced of where they were supposed to be. Which means, they launched way in advanced of any planned course heading and bearing they trained and practiced. You have to ask yourself, was there enough time for the flight desk to plot a new course after they were spotted by the Japanese fishing boat and the immediate launch of the B25s. If radio message reached Japan and they were ready, it was all over.
How did they find Tokyo, the course heading and distance would be completely different for dead reconning to work accurately. Mid flight course corrections would be necessary. No one had ever flown over Tokyo, they didn't know what landmarks to look for to see if they were on course or close by and it's all ocean so no landmarks or reference points. They were flying very low heights above the ocean to avoid detection and getting fired upon before reaching the capital. How could they even see where Japan was or know if they were headed the right way?
Without Celestial Navigation, they would not have known where was Tokyo. Dead reckoning works well if there is no drift and if you know where you started from, heading, and average velocity. You have a major problem if you don't know exactly the course you are supposed to follow.
"Doolittle and
Hornet skipper Captain
Marc Mitscher decided to launch the B-25s immediately—10 hours early and 170 nautical miles (310 km; 200 mi) farther from Japan than planned.
[note 6] After respotting to allow for engine start and runups, Doolittle's aircraft had 467 feet (142 m) of takeoff distance.
[34] Although none of the B-25 pilots, including Doolittle, had ever taken off from a carrier before, all 16 aircraft launched safely between 08:20 and 09:19, though Doolittle's bomber was witnessed to have almost hit the water before pulling up at the last second. The B-25s then flew toward Japan, most in groups of two to four aircraft, before flying singly at wave-top level to avoid detection.
[35]" -- Wikipedia
https://en.wikipedia.org/wiki/Doolittle_Raid