Conceptual artist Ryan Meinerding has released concept art that he created for a handful of films in the Marvel Cinematic Universe, such as Iron Man, Iron Man 2, Iron Man 3, Captain America: The First Avenger and Avengers: Age of Ultron.
The images include a sketch of Steve Rogers wearing his WWII rescue costume, Iron Man's Mark 43 armor, keyframe of Tony Stark testing his Iron Man boots, unused Mark 45 design for Age of Ultron, early War Machine design for IM2, and an early design of The Mandarin (aka Trevor Slattery).
Yes, I added one non-MCU image. Ryan posted an "armored version of Nite Owl (played by Patrick Wilson) from [Zack Snyder's] Watchmen that wasn't in the movie."
Check out Ryan's concept art in the images below.
Captain America: Civil War opens in theaters May 6, 2016.