For the most part, comic strips are created by people who are mainly known for creating comic strips. The lay person who is not well-versed in comic strip history may not even know the names of the creators of most of the comic strips they read, and if they do know them they don't know them for anything else. There are, however, some comic strips that were created by people who were famous for other, entirely unrelated things. Sometimes this is because they had a personal interest in creating a comic strip and in doing something different from what they had been doing. Other times it's because a newspaper syndicate wanted a big name in order to promote their new feature.
Such is the case with Dashiell Hammett.
Hammett is mainly known for his novels about hard-boiled detectives (a term supposedly coined by Tad Dorgan) such as The Maltese Falcon and The Thin Man. If you haven't read him, you certainly know the Humphrey Bogart version of his character Sam Spade from the movies. This was popular stuff in the 20s and 30s, and just like sci-fi adventures, it was beginning to leak over from the pages of pulp magazines onto the newspaper comics page. In 1931, Dick Tracy had begun in The Chicago Tribune, and was the biggest detective adventure comic at the time. After it had run for a few years and its popularity was apparent, King Features Syndicate wanted to have a feature to compete with it, much like they would do soon afterwards when they commissioned Flash Gordon to be created to compete with the Buck Rogers strip. In both cases, they hired Alex Raymond to do the artwork, though he was too obscure a name at the time. They needed a name on the feature that readers would recognize immediately, and thus immediately recognize the strip as a quality feature. They offered the job to Hammett, who, as luck would have it, was looking for new places to publish his work and readily accepted the offer.
The problem, however, was that it seemed King Features also wanted a writer they could control, and that was something Hammett was certainly not. He was used to writing stories about private detectives, which is why they hired him. For some reason, though, King Features really wanted a strip about a secret agent, not a private detective, and in order to reflect this there were substantial rewrites by the syndicate each time Hammett turned in his scripts. Because of this, it was very difficult to tell what exactly Secret Agent X-9 really was. He was a secret agent, but the agency he worked for was never revealed. He was also a private detective, but he only worked as one as a cover for his secret agent job. He acts like a detective and solves murders, yet the police don't seem to know who he is. It all ended up being quite the mess, and Hammett left the strip after less than a year. Raymond left a year after that, though he continued on several other King Features projects, including Flash Gordon, Jungle Jim, and his own comic strip about an actual private detective, Rip Kirby.
Interestingly, Secret Agent X-9 still continued under several other writers and artists for 60 more years, ending in 1996. After Hammett left, X-9 became a full-fledged secret agent. Later on, his name was revealed to be Phil Corrigan, and the name of the strip was even changed to "Secret Agent Corrigan". It was never extremely popular, but it is amazing that even after an incredibly shaky start it had quite a long run.
For more info:
Secret Agent X-9 at Toonopedia
Secret Agent X-9 at Thrilling Detective