The thing that started it was newspaper ink drying the instant it touched paper — lovely on a rotary press, hopeless in a fountain nib, too thick to flow. So he needed a way to meter it out, and here's the clever bit: it isn't really the ball, it's the socket round it, held to a few microns so the ink feeds and nothing leaks. Born 1899, and he was a journalist first, which is how he came to be standing near the presses at all.
Wendell, the RAF navigators were the ones who sold it — a fountain pen weeps at altitude when the cabin pressure drops, and the ball pen just carried on. Orders ran into the tens of thousands, which is a lot of pens for men doing sums on a knee board in a cold aeroplane.
The nicest bit of the story for a bookseller is what English did to his name: Bíró stayed capital and accented on the patent, and biro went lowercase in British usage within a generation, the way hoover and thermos did. A surname worn down into a common noun by ordinary hands — 1899 to a dictionary entry.