iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone X (GSM), released on 2018-04-24T17:09:40Z. The update package is 681.39 MB. This is a delta (partial) update that requires 11.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: c53461bb9ed6ac3eb53696496313d044fa7d4f4d.zip.