Joseph Baugher has a great reference on US Military Aircraft Designations. This explains the new and the old number series for attack (A), bomber (B), cargo (C) and fighter (F) aircraft.  It also describes when US Military unified the designations among the military branches and restarted the count for each category. (For example, B-2 Spirit flew a half-century after the B-17 Flying Fortress.)

GlobalSecurity.org also has an exhaustive list of US Military Aircraft. It includes Bomber, Attack, Fighter, Rotary (Helicopter), Cargo, X-Planes, Anti-Submarine, UAV, Tanker, Airborne Early Warning, Trainer, Utility and other Special types.