iPadOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPad Pro 9.7-inch (Cellular), released on 2018-04-24T17:08:34Z. The update package is 715.1 MB. This is a delta (partial) update that requires 11.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: c4f9b2550a8f43d10c83c52639ca9b61ed7c4513.zip.