What was the first commercial film shown in stereophonic sound?

In 1938, Disney started production of what would later be known as Fantasia. In January of that year, Leopold Stokowski recorded " The Sorcerer's Apprentice." RCA, working with Disney sound engineers using multiple audio channels which allowing any dynamic balance on playback. This early production was abandoned when, in the summer of 1939, David Sarnoff (founder of NBC and later President of RCA) agreed to create a sound system for the film. The collaboration between Disney and RCA led to the development of Fantasound a pioneering stereophonic surround sound system which innovated many of the processes widely used today, including simultaneous multi-track recording, overdubbing, and noise reduction.

With the release of the movie in November 1940, Fantasia became the first commercial movie to use stereophonic sound.

It has grossed nearly $80 million domestically and is ranked as the 22nd highest grossing film in the US after adjusting for inflation.