Earlier quoted context omitted.
They did it by ironic accident. Bill Gates wanted their browser to be the best, but also wanted it not good enough to replace desktop apps. However the right hand didn't know what the left hand was doing. The Outlook team was told to make a web version. They got the IE team to add XMLHttpRequest for their use, everyone implemented what they needed to, then went home and forgot about it. Then Google recognized what th…
Gmail was not even close the the first Ajax based email not by MS. Oddpost which later became Yahoo mail (Yahoo bought them to upgrade their mail) preceded Gmail by a few years. https://en.m.wikipedia.org/wiki/Oddpost
Furthermore I remember gmail's keyboard shortcuts being a shock to a lot of people. Me included.