iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 7 (GSM), released on 2018-04-24T17:11:33Z. The update package is 1.37 GB. This is a delta (partial) update that requires 10.3.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: f32ddefd7dfa79b4d2a0854531c72b76dd27df7f.zip.