Why is Captain Marvel a boy in the comics but is a girl in the movies?

Why is Captain Marvel a boy in the comics but is a girl in the movies? 

They are two different characters.

The original Captain Marvel was created in 1939 by Bill Parker and C.C. Beck for Fawcett Publications. The boy was named Billy Batson and he had the power to turn into Captain Marvel, the World’s Mightiest Mortal, by saying the magic word “SHAZAM!” which was also the name of the ancient wizard who gave him that power, and an acronym for the source of those powers (See the illustrations below).

       

There is a long and complicated story about the success of this character, a lawsuit that shut him down in 953, and how he came to DC comics, but after the lawsuit and before he came to DC comics, when he was not being published, Marvel Comics created their own Captain Marvel. This was in 1967.

This Captain Marvel was an alien warrior named Mar-Vell, a captain in his planet’s military. He disguised himself as a rocket scientist to get into Cape Canaveral and met Carol Danvers, head of security. In the course of their adventures, a machine from his planet exploded, and he thought he was protecting her by shielding her with his body.

In 1977, a new superhero, Ms. Marvel, appeared. It turned out that this was Carol Danvers. She had developed powers similar to Captain Mar-Vell due to a blending of his alien DNA with hers through radiation from the explosion.

Ms. Marvel had a long and tumultuous career, and Mar-Vell died in 1982 of cancer. There were several other Marvel characters who used the name Captain Marvel, some related to Mar-Vell and some not. In 2012, however, Mar-Vell was brought back to life, and then died again as he saved the universe.

After this, Carol Danvers decided to take on the mantle of Captain Marvel. This is the Captain Marvel who will be in the upcoming Marvel movie in 2019, played by Brie Larson.

Below are many of the appearances that Carol Danvers has had as a superhero, including as Ms. Marvel, Binary, Warbird, and Captain Marvel

    

Meanwhile, when DC licensed the original Captain Marvel from Fawcett in 1972, they could not use his name in the title because Marvel Comics had secured the trademark in 1967. Therefore, DC decided to title the comic, and use as the trademark for marketing, the magic word “SHAZAM!” This has led to confusion among people who never read the comic book as to the name of the hero, leading many to think his name was Shazam. In 2012 DC re-booted their entire line of comics, re-imagining many of their characters. One of them was this Captain Marvel, and this new version, with a new costume, new powers, and new backstory, although still the alter-ego of the boy, Billy Batson, is named Shazam. This is the hero who will be in the new movie coming out from DC in 2019, played by Asher Angel (as the boy) and Zachary Levy (as the hero).