Frank Borth took over the art for the Phantom Lady stories in Police Comics #17 (January 1943). Apparently he was not satisfied with her already-scanty costume (basically a one-piece bathing suit and a cape) and removed the back and added a split to her navel.
Despite there being more female costumed adventurers, most of them (like the Phantom Lady and Hawkgirl) seem to be simply an excuse to show more salacious imagery.
