Flight scenes were filmed in replica B-17s using technology known as The Volume (used on The Mandalorian (2019)). The B-17s were suspended 50 ft in the air on a gimbal inside a 360-degree stage of seamless LED panel screens and ceiling. Actors could therefore react to flak explosions, crashes and planes flying in real time as the gimbal simultaneously reacted to scenarios.
Three B-17 replicas were built from scratch for filming use. 2 were exact replicas, the other was slightly larger to allow more room for camera crew to film certain angles of the plane.
Sometimes the cast filming in the replica B-17s were left in the cockpit for up to 7 hours as it was so difficult to get the actors in and out of the suspended cockpit in The Volume.
The series was logistically a huge undertaking with over 3000 crew working on the 9 episodes. Steven Spielberg has said this is the biggest project he's worked on.
Austin Butler watched footage of the real-life Cleven who was interviewed about his time as a bomber pilot when he was in his 80s. The footage helped Austin learn Cleven's speaking cadence and sense of humour.