Not many people were mining with high-tech mining machines when Bitcoin was released (2009, if I'm not mistaken).
Thus, bitcoin was vulnerable to a 51% attack by a big company or an individual with huge resources.
But was there any such attack on Bitcoin around that time?
There were close calls that caused very high potential bitcoin 51% attacks. I don't believe that any were successful (definitely none successful enough to hurt Bitcoin as we are still here today). I understand from memory reading that there was a pool who had more than 51% of the hash power at one stage, though miners reallocated themselves and the pool made steps to reduce the hash power, I believe dividing into two pools was the approach.
I don't believe that a 51% attack is still something to worry about today if Bitcoin maintains its ability to make the globe compete for it's supply and its hash power.