Designed in 1949 by the American Engineer Robert Schwarz, the M65 “Atomic Annie” was inspired by German railway guns used during World War II. The M65 however, was designed to deliver a nuclear payload to its target.
The gun and carriage itself weighed around 85 tons, was manned by a crew of 5-7, and was transported by two specially designed towing tractors.
At 280mm in caliber and capable of firing a projectile over 20 miles, the gun was certainly powerful enough as a conventional weapon, but the Atomic Annie was certainly no conventional weapon.
In 1953 it was tested for the first time at the Nevada Test Site, where it fired a 15 kiloton nuclear warhead, creating a blast similar in size to the bombs dropped on Hiroshima and Nagasaki.
After the successful test, 20 M65 cannons were produced for the US Army and deployed in Europe and Korea. They were almost always in constant motion so the Soviets never knew where they were and could not target them. 
While an interesting weapon, the Atomic Annie suffered from limited range, especially after the development of ballistic missiles which could strike a target from thousands of miles away.
The last M65 Atomic Cannon was retired in 1963. Today only 8 survive, and are displayed in museums across the country.
You have a fire Mission? Okay, one on the way!
I stole this from that Fine Blog – The Daily Time Waster