Is Tomboy of Sterling Comics, from 1954, the first independent superheroine in comics?
"Superheroine" because she's a 10-year-old girl that can cling to the outside of a plane and jump into the water.
Unless that's actually possible, in which case the question becomes "Who's the real first?"
There are a considerable number of solo female superheroes that pre-date 1954.
One of the earliest in comics is Fantomah, previewing in Jungle Comics #2 (Feb, 1940) and then appearing in Jungle Comics #3 (March, 1940) in her own self-titled strip. Fantomah had a range of powers including the ability to command animals, sense what's happening in the jungle, change her appearance and cause fear.
We also have The Woman in Red, a masked crimefighter that debuted in Thrilling Comics #2 in March, 1940. She has no specific superpowers, but she does operate as a vigilante in much the same mould as Batman.
A solid contender is also Miss Fury, who first appeared in April, 1941 in a strip titled The Black Fury. Donning a magical catsuit, she gains super-strength and agility.
Definitely not first independent superheroine, but possibly first independent KID superheroine
If "independent heroine" means one who isn't a sidekick or derived female character (like Mary Marvel), then Wonder Woman (first appearance October 1941) well predates Tomboy.
However, according to this fandom wiki, "Tomboy is possibly the first female kid superhero (or to specify even more, heroine) who wasn't someone's sidekick."
I didn't realize it, but I was looking for kid superheroines. Even so, for the question I stated, I have found Olga Mesmer of 1937.
She has / had "phenomenal strength which [Olga's guardian] has been in mortal dread for years" along with her titular 'x-ray eyes'.
