It was the first of the two nuclear weapons that were ever used in warfare.
The Mk I "Little Boy" was 10 feet (3 m) in length, 28 inches (71 cm) wide and weighed 8,900 lb (4000 kg). The design used a gun arrangement to explosively force a sub-critical mass of uranium-235 and three U-235 target rings together into a super-critical mass, initiating a nuclear chain reaction. The yield of "Little Boy" was about 13 kilotons of TNT equivalent in explosive force, i.e. 5.5×1013 joule = 55 TJ (terajoule). Approximately 75,000 people were killed as a direct result of the blast, though more died later as a result of fallout and cancer.
At the time there had never been a test explosion with this type of weapon. The only test explosion of a nuclear weapon was with the plutonium-type, on July 16, 1945 at the Trinity site. This was because tests of controlled nuclear reactions with U-235 (as opposed to the uncontrolled reaction that occurs in a bomb) had already been done, and the principles involved were so simple that it was taken to be unnecessary to test the weapon in advance. The military were also anxious to drop the bomb, and testing the device would have delayed its use until more uranium was ready.
Although used occasionally in later experimental devices, the design was used only once as a weapon because of the extreme danger of a misfire. A simple crash could drive the "bullet" into the "target" and release lethal radiation doses or even a full nuclear detonation. The danger of misfire was even greater over water. Even if the force of a crash did not set the bomb off, if water entered the fail safe system, it would be shorted out, leading to a detonation of the bomb. The British Red Beard nuclear weapon also suffered from this flaw.