It was a plan to show the Japanese the USA was not knocked out by the raid on Pearl Harbor.
Jimmie Doolittle, a long-time supporter of Naval aviation and long-range bombers took a hand-picked group of pilots to the desert. They practiced getting their heavily-laden Liberators [I think] into the air in a very short distance.
At first, he didn't explain what it was all about.
It turned out that he loaded those bombers onto a US Navy aircraft carrier [should do a Google to get the name but am too lazy right now]. They sailed as close as possible to Japan but had to take off early as a Japanese ship spotted them. They flew and dropped bombs on Tokyo. They then continued to the Chinese mainland. A lot crashed and some were caught by Japanese troops - some were saved by the Chinese.
The raid did its purpose - to raise the morale of the American people and show the Japanese that, while they had created havoc at Pearl Harbor, we were not down and out.
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irate2:
In fact, Yamamoto, the Japanese brain behind Pearl Harbor, always worried about the results and he had not sunk the American aircraft carriers.
It was a milestone in proving just how powerful they were in a time when most admirals thought only battleships and heavy cruises could carry the war forward.
[didn't mean to get so long-winded]