 The gun @-@ type and implosion @-@ type designs were codenamed " Thin Man " and " Fat Man " respectively . These code names were created by Robert Serber , a former student of Oppenheimer 's who worked on the Manhattan Project . He chose them based on their design shapes ; the Thin Man would be a very long device , and the name came from the Dashiell Hammett detective novel The Thin Man and series of movies by the same name ; the Fat Man would be round and fat and was named after Sydney Greenstreet 's character in The Maltese Falcon . Little Boy would come last , as a variation of Thin Man . 
