According to the 1999 book Batman: The Complete History, by Les Daniels, Bob Kane's inspiration for Batman included Leonardo da Vinci, Zorro and the 1930 mystery movie The Bat Whispers. But clinical psychologist Richard A. Warshack says that Kane's gritty detective was inspired by something far more personal and painful.

In an essay in the Atlantic, Warshack suggests that Kane was suffering from post-traumatic stress disorder.

Warshack draws upon an incident from Kane's autobiography, Batman and Me, when he was 15, in 1930, in the Bronx. Young Bobbie Kahn (as he was known before he changed his name) was walking past a gang who called themselves the Vultures while carrying a violin. After yelling that only girls played the violin, the Vultures chased him down into a lumber yard.

Kane wrote that he had fended them off with a grappling hook. But soon, the Vultures took him down.

I was in a fog, when I felt my right arm crack at the elbow after a gang member deliberately twisted it behind me in order to break it. The pain was excruciating and I screamed in agony. Before I blacked out and fell to the ground like a limp rag doll, I heard him laugh sardonically, “Just to make sure dat da Fiddler ain’t gonna play his fiddle no more!” Little did he know that it wasn’t playing the violin again that concerned me, but the fact that he had broken my drawing arm. Then he stepped on the hand of my broken arm! ... This whole episode, to this day, remains in my subconscious like a nightmare.

While Kane later reveals that he should have known better than to carry a violin in a rough part of the Bronx, Warshack explains that this very statement is known as "omen formation."

[A]nother common symptom of trauma victims—finding reasons why he should have known in advance about the danger or suspected that something bad would occur. … So, according to Kane, the beating he suffered was not because he had the ill fortune to be accosted by a gang of hoodlums. It was something he could have predicted. It was in his control all the time … He chose the wrong instrument. This explanation, I believe, is Kane’s defense against the terror of being helpless.

In other words, it's clear to Warshack that Kane had suffered from post-traumatic stress disorder. In fact, it's the kind of incident that Bruce Wayne suffers when he witnesses the death of his parents.

Kane, who was hospitalized as a result of that beating, later wrote Batman as a dark avenging knight and became one of the most famous comic-book creators of all time--all "thanks" to his trauma.

Via The Atlantic.