Bill vs Billy

Bill

name
  • A diminutive of the male given name William. 

  • A nickname for the British constabulary. Often called "The Bill" or "Old Bill" 

  • A surname. 

Billy

name
  • A diminutive of the male given name William. 

  • The B-25 twin-engine bomber aircraft used during World War Two, commonly called the "B-25 Mitchell" in honor of U.S. Army General William "Billy" Mitchell. 

How often have the words Bill and Billy occurred in a corpus of books? (source: Google Ngram Viewer )