D.B. is Holden's older brother is a screenwriter in Hollywood. He used to write great stories – so great, in fact, that Holden credits him (twice) with being his favorite writer. D.B. is the height of phoniness in Holden's mind because he's sacrificed his art (writing stories) for money (writing screenplays). Compare this, by the way, to Estelle Fletcher, who avoided sounding cheesy and phony in her Little Shirley Beans record. D.B. is just the opposite – at least according to Holden. In short, he's a sell-out.
But the most telling information we get about D.B. has to do with the war. Holden reveals in a flashback that D.B. used to be in the Army. From what the narration reveals, the experience had quite a traumatizing effect on D.B. When he would come home on leave, Holden says, he used to lie on his bed the whole time and stare at the ceiling. His response to Allie that being in the war "doesn't do a damn thing to help his writing" has a bitterness and melancholy to it. His statement to Holden that the Army is as full of "bastards" as the Nazis, and his claim that if he did have to shoot he wouldn't even know where to point his gun, argue for the absurdity of war. When Holden has a comparable opinion on the atomic bomb – that he'll volunteer to sit on top of the next one – we see that, despite his younger age, he too has similar feelings on the war.
D.B. reminds us, then, that The Catcher in the Rye has a lot to do with the war (WWII that is). This is 1949 (or 1948) after all – how could it not? We talk about this in greater detail in the Setting section, but D.B. makes it personal, renders human the effects of the historical events. We can talk generally about isolation and depression and disillusion, but it takes the image of D.B. lying on his bed and staring at the ceiling to remind us what this all means to a person living through it.