Even if we ever develop the technology to do this, it would create a paradox. http://en.wikipedia.org/wiki/Grandfather_paradox
"A variant of the grandfather paradox is the Hitler paradox or Hitler's murder paradox, a fairly frequent trope in science fiction, in which the protagonist travels back in time to murder Adolf Hitler before he can instigate World War II. Rather than necessarily physically preventing time travel, the action removes any reason for the travel, along with any knowledge that the reason ever existed, thus removing any point in travelling in time in the first place.[5] Additionally, the consequences of Hitler's existence are so monumental and all-encompassing that for anyone born in the decades after World War II, it is likely that the grandfather paradox would directly apply in some way (say, if your great-grandparents were Holocaust refugees, or even if the train your parents met on was constructed as part of the war effort)."