Robert Downey Jr. based his character on Dr. William Price, an eccentric Welshman. Dr. Price is now a revered historical figure in Wales - there's even a statue of him in his hometown of Llantrisant.
Originally titled "The Voyage of Doctor Dolittle." It was shortened in August 2019.
This movie was originally going to be released on May 24, 2019, by Universal Pictures, but was moved to April 12, 2019, to avoid competition with Star Wars: Episode IX - The Rise of Skywalker (2019). It was later pushed back again to January 17, 2020. This concerned many because January is when studios have historically released movies they feel will fail. The report that this movie went through almost a month of re-shoots and re-writes supported this concern.
When the movie rights went up for auction, several studios went to bid before Universal Pictures got the rights. Amongst them were Sony and Twentieth Century Fox, with the latter having previously produced three theatrical movies based on the Dr. Dolittle series.
To differentiate from his earlier British characters, Robert Downey Jr. decided to use what he described as a "passable" Welsh accent for his version of Dr. Dolittle.