On his birth planet, Krypton, gravity was much stronger than it is on Earth, so Kryptonians

Kryptonians

Kryptonians are a fictional extraterrestrial race within the DC Comics universe that originated on the planet Krypton. The term originated from the stories of DC Comics superhero, Superman.

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developed a physique that could resist its pull. When he's on Earth, Superman's physique can defy the planet's weaker pull easily, and that ability, enhanced by solar energy, allows him to fly.

Was Superman always able to fly?

When he came to life on the pages of Action Comics #1 in April of 1938, our hero boasts myriad superhuman abilities but initially lacked one of the most iconic of them: flying. It wasn't until a radio program in 1940 that Superman could take to the sky in the United States — and be confused with birds and planes.

Is Superman the only Kryptonian that can fly?

Originally Kryptonians weren't capable of flying on Krypton because it orbited a red sun, which depowers Kryptonians. Decades later in comic books Krypton was revived as New Krypton which orbited a yellow sun giving all Kryptonians the ability to fly.