The original Little Boy and Fat Man weapons, developed by the United States during the Manhattan Project, were relatively large (Fat Man had a diameter of ) and heavy (around 5 tons each) and required specially modified bomber planes to be adapted for their bombing missions against Japan.
Through the aid of brainpower acquired through Operation Paperclip at the tail end of the European theater of World War II, the United States was able to embark on an ambitious program in rocketry.