Batman is an ongoing American comic book series featuring the DC Comics superhero Batman as its main protagonist. The character first appeared in Detective Comics # 27 (cover dated May 1939). Batman proved to be so popular that a self-titled ongoing comic book series began publication with a cover date of Spring 1940. It was first advertised in early April 1940, one month after the first appearance of his new sidekick, Robin, the Boy Wonder.

Answer this question based on the passage: when was the first batman comic book published? Okie... think carefully first, then make a decision:
To answer the above question, the relevant sentence is: Batman proved to be so popular that a self-titled ongoing comic book series began publication with a cover date of Spring 1940. So, the answer is Spring 1940.