He started flying in the 1940s right after the creation of the Fleischer films. This was because it became difficult so show him leaping from place to place. The Fleischer films used a form of rotoscoping and many of Superman's actions had to be drawn by animators because they couldn't be rotoscoped at the time. It was easier to show him flying than leaping.
The Fleischer cartoons were also responsible for giving Superman perhaps his most singular superpower: flight. When the Fleischers started work on the series, in the comic books, Superman could only leap from place to place (hence the classic phrase, "able to leap tall buildings in a single bound"). After seeing the leaping fully animated, however, the Fleischers deemed it "silly looking", and asked Action Comics' (which would later become DC Comics) permission to have him fly instead; the publisher agreed, and wrote the flight ability into the comics from then on. REF: Wikipedia > 1940 Superman Films
Superman officially first flew in a comic book in Action Comics #65, released in October 1943:
Superman's pedigree of powers has grown and varied over the decades. Since he is one of the oldest, if not the oldest of the four-color heroes, it is difficult to tell exactly when a particular power is considered to have come into fashion.
His powers when he first appeared in Action Comics #1 in 1939 were:
- Leaping one-eight of a mile
- Nothing less than a bursting shell could penetrate his skin
- Strong enough to lift an automobile of the time, cleanly over his head
- He had no superhuman visions, senses or ranged abilities
- Depending on what you read, all Kryptonians might have had similar powers on Krypton and were a race of genetically advanced humanoids.
While this was sufficient for the comic strip for a while, it became a problem for the Superman when he became a radio play character and even more so when he became animated during the early 1940s.
The Fleischer & Famous Superman animated shorts are some of the finest cartoons of the period. During this time, Superman became a bit stronger and a bit faster.
He became strong enough to lift giant robots
- He became fast enough to race locomotives
- He didn't quite fly, but could leap long distances and it looked like flying...This may be the very first examples of leaping become directional flight.
- It was a subtle transition but as the episodes progressed you saw it more and more.
In the early radio programs and even from the very first Fleischer film Superman (aka The Mad Scientist) Superman was supposedly "leaping" from place to place. But artistically having to show our hero jumping around was stylistically difficult given the film lengths and effort required to make them.
So his leaps would transition from a jumping movement where he would use his running momentum to jump and his hands to point skyward. This often transitioned into a swimmer's pose with his hands at his sides as he would begin to "fly" and gain speed.
It was from this position he would streak to his target "like a speeding bullet." Look at the Mad Scientist from 6:36 to 6:40 showing a jumping to flight transition.
During this time, superheroes were beginning to gain traction and Superman was experiencing competition. So he became a bit flashier to keep up. During the fifties, the Superman Family expanded adding Krypto, Supergirl and Mon-el.
At this point Superman's powers began to expand:
- adding a range of sensory abilities, super-hearing, x-ray vision, telescopic vision
- also started using his x-ray vision as a weapon eventually becoming "heat vision"
- his strength became significantly greater in the fifties, keeping him the biggest dog on the superhero landscape which had begun to grow.
- He began using his cape as a tool, since it was also invulnerable, as everything from Krypton was.
- His flight firmly established in all mediums now, he used his cape to contain explosions, and protect humans from his superhuman flight on the way to his Fortress of Solitude in the Arctic.
- Images from the period showed less of the leaping motif and more of the flying with the arms out front.
He would often, however, have to deflect materials, bullets, shells, deadly beams of energy and would use his hands to do so. This lead to a pose where he would have at least one of his hands out in front of him to catch or deflect incoming attacks. He would also do this if he were carrying someone to protect them as well.
This ultimately lead to the later flying scenes where he would be shown flying and artists treated his flying as if it were a form of anti-gravity swimming where he would alter his direction by twisting his body and changing his leading pointing hand.
Superman's transition to flight was a slow procession from his comic strip.
With each medium he was transferred to his powers increased, his range of abilities increased and eventually he developed the power of flight, even though no one has ever sufficiently bothered to explain how it works.
I would blame the Fleischer & Famous Superman animated short films for the development of the power to ease the story development and move things along in an expeditious manner. Watch the films and you can see how it happens.