As I said above - and perhaps you might like to try a primary source rather than wikipedia ?

  These weapons were, however, as different as their monikers imply. Little Boy was a uranium, gun-type weapon, whereas Fat Man was a plutonium, implosion-style weapon. Two types were needed because there was only enough uranium for one gun-type weapon, and the U.S. government knew it would need to make more than one weapon. 

~ Los Alamos Nuclear Lab https://www.lanl.gov/media/publications/the-vault/1023-a-tal...