The dress itself was inspired by Marilyn Monroe's 1951 Oscars dress, as well as Holiday Barbie gowns that Billie obsessed over as a kid.
The look completed something of an evolution for the 19-year-old, who began to transform her style earlier this year when she debuted a new look on the cover of British Vogue.
However, although the dress seemed light-years away from the style that Billie has become known for, the Grammy-winning singer revealed days before the event that she always loved wearing dresses as a kid and only stopped wearing them because body image "tore that shit down."
"Big dresses were my favorite thing when I was a kid," she explained in a behind-the-scenes video with Vogue in the lead-up to the Met Gala. "I had so many dresses. I would wear a dress every single day. It's really body image that tore that shit down. Why do you think I've been dressing the way I have for years?"
This related back to something Billie said in 2019 when she explained why she often wore clothes "800 sizes bigger" than she actually was.
"It kind of gives nobody the opportunity to judge what your body looks like," she told Vogue Australia. "I want layers and layers and layers and I want to be mysterious. You don't know what's underneath and you don't know what’s on top."
She later explained that she'd drawn inspiration from Rihanna, saying: "I remember seeing an acceptance speech that Rihanna gave where she said that fashion has always been her defense mechanism. And I feel like I've never felt something as strongly hearing someone else say it than I did when I heard her say that. Because it's always been that way for me. It’s always been my security blanket."
"I feel like I've grown so much in the last few years, and my confidence has gotten so much better," she added. "I've always wanted to do this. I was just scared and didn't feel comfortable in my skin."
In fact, Billie had planned to wear a similar dress before COVID canceled last year's Met Gala, although she'd originally wanted it to match her then–acid green hair.
"I was planning on thinking of something for the Met of 2020, and I was like, 'It should be bright green, and it's gonna be some stupid print on it,'" she said. "I don't know, but I knew that I wanted something big with a corset."
Dena Giannini, the style director for British Vogue and the person in charge of styling the 19-year-old for the event, elaborated on this point, explaining that Billie had "always dreamed of wearing a proper ballgown — particularly something corseted, thanks to her love of Barbies growing up."
But Dena noted that Billie's "style began to evolve" after her British Vogue cover, which is how the dress transformed from its original concept.