After losing in a fistfight with Thanos in the prologue to Avengers: Infinity War, Bruce Banner never transforms back into his alter ego as the Hulk for the remainder of the film, save for a pair of brief, angry outbursts which never quite result in the Jade Giant's return.
However, rough concept art included in the film's art book, Marvel's Avengers: Infinity War - The Art of the Movie, reveals that the Hulk was originally slated to appear in the climactic Battle of Wakanda as part of the ensemble's efforts to defeat Thanos in their failed bid to prevent the Mad Titan from using the Infinity Gauntlet to wipe out half of all sentient life in the universe.
The early idea would have taken place late in the film after the arrival of Thor, Rocket Raccoon and Groot on Earth, as well as Thanos himself, in the battle's final moments. The art shows Banner is no longer using the Hulkbuster armor as he does in the entire battle in the released movie, instead shifting completely to his rage-powered alternate persona, holding down Thanos' right arm while Steve Rogers, Black Widow (with her usual red hair), Black Panther and Groot also try to restrain the villain. Meanwhile, Thor attempts to deliver a killing blow, though even here he's not aiming for Thanos' head.
Amidst the chaos, we see Wakanda's Dora Milaje joined by Rocket and War Machine in fighting Thanos' savage Outriders, though the film has the extraterrestrial army largely wiped out by the time their purple overseer arrives on Earth. Additionally, the theatrical film ultimately had the heroes in Wakanda face Thanos individually, one after another, as opposed to the group effort seen in the concept art. This was presumably changed to avoid comparisons with the superhero team-up on Thanos' homeworld of Titan and to heighten the dramatic buildup in the film's climax.
There were longstanding rumors that the Hulk would play a pivotal role in the Battle of Wakanda leading up to the release of Infinity War, fueled by appearances of the strongest Avenger as part of the final charge seen in the film's trailers and promotional clips. Co-director Joe Russo revealed after the film's release that the idea of Banner being saved at the last minute by his alter ego had become predictable, and the decision was made to empower the character instead by having him rely on himself within the Hulkbuster armor instead. Thus including the Hulk in promotional materials from Wakanda was designed to consciously mislead audiences and surprise them going into the final film.
Given how poorly the Hulk fared going against Thanos mano-a-mano in the film's prologue, it is unclear how effective he would have been against the Mad Titan in Wakanda for a rematch with friends and allies in tow. However, the concept art does show that the climax was not completely set in stone from the start. It will be interesting to see how an inevitable rematch between Hulk and Thanos unfolds in Avengers 4 next year.
In the meantime, Marvel's Avengers: Infinity War - The Art of the Movie and the film are both out now.