Bootstrap, originally known as Twitter Blueprint, was created by Mark Otto (left) and Jacob Thornton (right).
It was orginally made as an internal Twitter style guide before being released to the public as Twitter Bootstrap on August 19th, 2011. When Bootstrap was being used at Twitter, it served as a style guide for the website, as opposed to larger libraries that were more common at the time.
Large libraries were hard to work with and the consistency between elements was not always great, thus, Bootstrap was created.
According to Mark Otto:
"A super small group of developers and I got together to design and build a new internal tool and saw an opportunity to do something more. Through that process, we saw ourselves build something much more substantial than another internal tool. Months later, we ended up with an early version of Bootstrap as a way to document and share common design patterns and assets within the company."