Not many people were mining with high-tech mining machines when Bitcoin was released (2009, if I'm not mistaken).
Specialized hardware for mining (ASIC) didn't exist in 2009, everyone was mining with CPU.
Thus, bitcoin was vulnerable to a 51% attack by a big company or an individual with huge resources.
Theoretically it's possible, but who would bother attack Bitcoin at that time? Nobody even bother to create bunch of transaction to spam blockchain at that time.
But was there any such attack on Bitcoin around that time?
As far as we know, no.