According to Marvel’s Chief Creative Officer Kevin Feige, Black Widow will explore parts of Natasha Romanoff’s past that will make viewers see certain events from Avengers: Infinity War and Avengers: Endgame in a “new light.”
At Brazil Comic Con (CCXP19), IGN Brazil asked Feige when he and his team decided to put Black Widow between Captain America: Civil War and Avengers: Infinity War instead of it possibly being a prequel for the first Avengers or something similar.
“It was about four years ago as we were working on Infinity War and Endgame at the same time, we knew that we wanted to bring her story to a conclusion in the most heroic way possible in Endgame, but also that we wanted to explore a part of her life that we hadn’t seen before,” Feige said. “It occurred to us that we had seen her adventures in the Avengers movies, but a lot of stuff happened in between those movies that we never saw, we didn’t hear about, we didn’t learn about. And
Source: IGN.com Kevin Feige on How Black Widow Will Change How You See Infinity War and Endgame