Girl Wearing Superhero Panties

The first known female superhero is writer-artist Fletcher Hanks's minor character Fantomah, an ageless, ancient Egyptian woman in the modern day who could transform into a skull-faced creature with superpowers to fight evil; she debuted in Fiction House's Jungle Comics #2 (February 1940), credited to the pseudonymous "Barclay Flagg".
Another seminal superheroine is Invisible Scarlet O'Neil, a non-costumed character who fought crime and wartime saboteurs using the superpower of invisibility; she debuted in the eponymous syndicated newspaper comic strip by Russell Stamm on June 3, 1940. A superpowered female antihero, the Black Widow — a costumed emissary of Satan who killed evildoers in order to send them to Hell — debuted in Mystic Comics #4 (August 1940), from Timely Comics, the 1940s predecessor of Marvel Comics.
Though non-superpowered, like the Phantom and Batman, the earliest female costumed crimefighters are The Woman in Red, introduced in Standard Comics' Thrilling Comics #2 (March 1940); Lady Luck, debuting in the Sunday-newspaper comic-book insert The Spirit Section June 2, 1940; the comedic character Red Tornado, debuting in All-American Comics #20 (November 1940); Miss Fury, debuting in the eponymous comic strip by female cartoonist Tarpé Mills on April 6, 1941; the Phantom Lady, introduced in Quality Comics Police Comics #1 (August 1941); and the Black Cat, introduced in Harvey Comics' Pocket Comics #1 (also August 1941). The superpowered Nelvana of the Northern Lights debuted in Canadian publisher Hillborough Studio's Triumph-Adventure Comics #1 (August 1941), and the superhumanly strong Miss Victory was introduced in Holyoke (comics) the same month. The character was later adopted by A.C. Comics.
The first widely recognizable female superhero is Wonder Woman, from All-American Publications, one of two companies that would merge to form DC Comics. She was created by psychologist William Moulton Marston with help and inspiration from his wife Elizabeth and their mutual lover Olive Byrne. Wonder Woman debuted in All Star Comics #8 (January 1942).