Billy vs Willie

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. 

Willie

name
  • A diminutive of the male given name William 

  • A diminutive of the female given name Wilhelmina or a feminine diminutive of William, often in the form Willie Mae. 

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