"It's always been my dream to play Aretha, and it's actually happening," she told host Ellen DeGeneres.
The Voice coach confirmed Franklin handpicked her for the film, which is in early production.
"It blows my mind," she said. "I've always looked up to Aretha and to portray her -- I can't even speak about it because it's like, 'Oh, that's a huge task.'"
Hudson said the movie will span the Queen of Soul's illustrious, decades-long career.
"It's not just two, three decades of a career -- this woman had a lifetime of a career. That's like 70 years. I can't even begin to imagine how long it's going to take to cover," she said.