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. Each modified bomber could only carry one such weapon and only within a limited range. After these initial weapons were developed, a considerable amount of money and research was conducted towards the goal of standardizing nuclear warheads so that they did not require highly specialized experts to assemble them before use, as in the case with the idiosyncratic wartime devices, and miniaturization of the warheads for use in more variable delivery systems.