Aircraft
      by Nation
      United States
      Navy/Marine Corps (1923 - 1962)
      Patrol: The
      "P" for patrol designation is one of the original 1923
      designations and is still in use today.  In the late-1930s through
      the end of World War II, patrol aircraft that carried their own bombs
      acquired the designation "PB" for Patrol Bomber.  Post-war
      practice reverted to the single-letter "P" designation.