B.O.B. was the butler of Elizabeth Ashe's family. Her parents paid little attention to her, and care of their daughter was mostly relegated to B.O.B. Thus, B.O.B. was there when Ashe's parents weren't. He was there when she partook in target shooting on the family estate, and was there when Ashe was released from prison. Throughout her youth, Ashe spent a lot of time talking to B.O.B., even though he apparently couldn't talk back.
On the day of her graduation, Ashe was arrested yet again, causing her parents to disinherit her. She teamed up with B.O.B, intent on recovering her fortune. When Ashe founded the Deadlock Gang, B.O.B. remained at her side and partook in heists himself. At some point after joining the gang, his chasis was fitted with armour plating.
At some point after Jesse McCree left the gang, Ashe had B.O.B. upgraded with arm cannons.
Sorry about that, B.O.B. No hard feelings?
~ McCree after decapitating B.O.B.
B.O.B. and other gang members took part in a heist on Route 66. A military cargo train was derailed, and they began to help themselves to the spoils, before McCree arrived on the scene. He told the gang that they could help themselves to the spoils bar a single crate, which he wanted for himself. This piqued Ashe's interest, and she ordered B.O.B. to open the crate. He obliged, and both servant and master looked at the contents with interest. Ashe told McCree to tell her what it was, but he refused, and a standoff developed, followed by a gunfight. Despite being outnumbered, McCree quickly incapacitated most of the crew, though grabbing a Route 66 signpost, B.O.B. was able to deflect some bullets away from Ashe. His master yelling at him to do something, B.O.B. activated his arm cannon and opened fire, forcing McCree to take cover. However, McCree threw some grenades their way and used his revolver to detonate them. The resulting blast separated B.O.B.'s head from his body.
In the aftermath, McCree loaded the gang members on a hover dolly, sending them down the road. He hoped there were no hard feelings between him and B.O.B., to which the omnic simply blinked. To add insult to injury, McCree took off on Ashe's own hovercycle, heading straight past them. After venting her frustrations, Ashe asked B.O.B. what he was looking at, a question to which again, the omnic remained silent.
B.O.B.'s name stands for "Big Omnic Butler," but can also stand for "Big Omnic Bodyguard" or "BOB, Omnic Butler."
Michael Chu has joked that B.O.B. can be considered "hero 29.5" (Ashe being the 29th hero of the game).
B.O.B. is a rare exception among characters in Overwatch, having required virtually no iteration before reaching a finalized design that satisfied the team, with Jason Hill comparing him to a unicorn and saying "B.O.B. was just meant to be." The initial pitch for B.O.B. came from Jason Hill; up to the point of his design, the only omnic types had been the standard humanoid omnic or Bastion. He reasoned that omnics "should come in all shapes and sizes." The artwork initial as done by Mio, while the concept was refined by Jungah Lee.
↑Ashe, Blizzard Entertainment. Accessed on 2018-11-13