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.