Batman: A Visual History
1. Detective Comics #27
The first comic Batman ever appeared in was Detective Comics #27 in May of 1939. In this comic, you can see Batman's costume has the classic elements (though it is a bit camp looking) but his personality was different to how he's been portrayed more recently.
"The Case of the Chemical Syndicate" was written in the style of old pulp comics and as a result, Batman showed little remorse for killing at this time.