The Department of Defense decided that they wanted an atomic capable artillery piece in 1949 and chose a engineer named Robert Schwartz to design it. The Atomic Annie was created by Picatinny Arsenal and was designed by essentially scaling up the largest cannon in the arsenal, the 240mm, and using the German K5 Railroad gun as a basis for its carriage.
First fielded in 1953, eventually 20 of these monsters were manufactured and they were deployed to bases in Germany and Korea to threaten the commie hordes. The gun sites were reportedly moved often to avoid being targeted by enemy forces.
The M65 was rendered obsolete fairly quickly as the DoD soon created nuclear artillery shells that were compatible with standard 155mm and 203mm cannon, and later the invention of heavier rockets, and eventually the ICBM. But since it was so sweet and military planning in that era was very conservative and fully expecting Ivan to sweep into Western Europe with thousands of tanks it remained in service until 1963.
The M65 was transported by two specially designed 375 horsepower tractors, with independent steering like some very long fire engines. The result did not look very graceful, but it could get up to 35mph and make turns on a 28 foot wide road. The Atomic Annie could be unlimbered in 15 minutes and then returned to traveling configuration in 15 minutes more. The name "Atomic Annie" is said to be related to "Anzio Annie" the name of a K5 gun that was used against the US Army landings in Anzio, Italy during WWII.
On May 25, 1953 at 8:30am local time, the Atomic Cannon was tested at the Frenchman Flat of the Nevada Test Site as part of the Upshot-Knothole series of nuclear tests. The test--codenamed Grable-- resulted in the successful detonation of a 15 kt shell (warhead W9) at a range of 7 miles. This was the first and only nuclear shell to be fired from a cannon.