What was the first iPhone model with a 5.5-inch display?

The first iPhone model with a 5.5-inch display was the iPhone 6 Plus.

Apple unveiled the iPhone 6 Plus alongside the 4.7-inch iPhone 6 on September 9, 2014. Both models represented a major shift from Apple's earlier iPhones, which had used smaller screens: the iPhone 5 generation had a 4-inch display, while earlier models measured 3.5 inches.

The iPhone 6 Plus used a 5.5-inch Retina HD LCD with a 1920-by-1080 resolution and 401 pixels per inch. Its larger size also enabled features such as a two-column landscape layout in some apps and a Reachability gesture to bring the top of the screen closer to the user's thumb.

The iPhone 6s Plus, iPhone 7 Plus, and iPhone 8 Plus also used 5.5-inch displays, but they came later. The iPhone 6 Plus is therefore the first model in that larger “Plus” line and the correct answer.