Superman #1 (Jun. 1939)
Here, his home planet of Krypton is officially named, and the Kents are introduced (with Pa's name not mentioned, but Ma named Mary). I always loved the humor in the second and third panel: the Kents are nonchalant in stumbling on a baby deserted in a rocket, while baby Superman lifting the dresser (and the hapless attendant) is Shuster at his funniest.
But Pa Kent is a little dark in this, telling a young Clark to hide his strength from others, "or they'll be scared of you!" Ma, on the other hand, encourages Clark to use his great power "to assist humanity."
It's the death of his foster parents in adulthood--after he goes through life honing his ability to leap an eighth of a mile, lift cars over his head, and racing express trains--that spurs him to vow by their graves to channel his powers into serving mankind as Superman.
The next four pages set up Superman's assault on the Governor's Mansion from Action Comics #1, by establishing the murderess on the lawn as the story that gets Clark his job at the Daily Star.
Batman's origin was, likewise, just two pages, told in broad strokes. It feels like 1940 brought out the dedicated origin story of eight pages or more. Had Superman and Batman come out a year or two later, I wonder if their origins would have been more in that vein.