The character of "Rosie the Riveter"—as feminist symbol, World War II icon and mid-century heroine—is so ingrained in the American psyche that it's sometimes difficult to remember that there was a time when Rosie didn't, in fact, exist. In the early 1940s, though, as American women flooded the labor force in order to replace the millions of men who had gone off to war, a wide variety of songwriters, illustrators—like the Saturday Evening Post's Norman Rockwell—and photographers effectively invented the archetype on which all subsequent Rosies were based.
(Pittsburgh artist J. Howard Miller's famous 1942 "We Can Do It!" poster, created for Westinghouse House and featuring easily the most famous and recognizable "Rosie" of them all, was not widely known during the war years, and only assumed its current, iconic status decades later.)
Among the photographers who documented this massive and, in a very real sense, revolutionary influx of female workers into traditionally male factory jobs—as welders, lathe operators, machinists and, of course, riveters—was LIFE's Margaret Bourke-White.
A pioneer herself (one of LIFE magazine’s original four staff photographers, America’s first accredited woman photographer during WWII, the first authorized to fly on a combat mission, etc.), Bourke-White spent time in 1943 in Gary, Indiana, chronicling "women ... handling an amazing variety of jobs" in steel factories — "some completely unskilled, some semiskilled and some requiring great technical knowledge, precision and facility," as LIFE told its readers in its August 9, 1943, issue. The magazine went on to note:
In 1941 only 1% of aviation employees were women, while this year they will comprise an estimated 65% of the total. Of the 16,000,000 women now employed in the U.S., over a quarter are in war industries. Although the concept of the weaker sex sweating near blast furnaces, directing giant ladles of molten iron or pouring red-hot ingots is accepted in England and Russia, it has always been foreign to American tradition. Only the rising need for labor and the diminishing supply of manpower has forced this revolutionary adjustment.
The women are recruited from Gary and nearby East Chicago. A minority has drifted in from agricultural areas. They are black and white, Polish and Croat, Mexican and Scottish... The women steel workers at Gary are not freaks or novelties. They have been accepted by management, by the union, by the rough, iron-muscled men they work with day after day. In time of peace they may return once more to home and family, but they have proved that in time of crisis no job is too tough for American women.
Here, LIFE.com presents a series of pictures from the Gary mills in 1943, in the very midst of the Second World War. Here are portraits of individual women, pride shining from their faces, as well as characteristically marvelous Bourke-White shots of enormous machines, grease-lathered gears, powerful tools—photographs that capture the grit, grime and rugged, unexpected beauty of a factory and its workers in full production mode.
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter at @LizabethRonk