Rudolph the Red-Nosed Reindeer made his first appeaarance in 1939 in a story that was written to what?

Rudolph the Red-Nosed Reindeer made his first appearance in 1939 in a story written by Robert L. May as a promotional piece for the Montgomery Ward department store. The story was intended to provide a Christmas-themed coloring book for children, and it introduced Rudolph as a reindeer with a glowing red nose who ultimately helps Santa Claus deliver gifts on a foggy Christmas Eve.