1947 – At Cincinnati, Ewell Blackwell whips the Cubs, 5 – 1, to start a 16-game winning streak. He won’t lose until July 25.

On May 10, 1947 — 1947 – At Cincinnati, Ewell Blackwell whips the Cubs, 5 – 1, to start a 16-game winning streak. He won’t lose until July 25.

Source:
Baseball Reference May 10