From Memory Alpha, the free Star Trek reference.
"Gene Roddenberry wanted a character named "Rice" in each TV show he created (see Lt. William Rice in "The Lieutenant"), so the Captain's name was going to be "James Rice Kirk". This explains the "James R. Kirk" tombstone Mitchell creates for Kirk. As the series developed after the second pilot it was somehow forgotten and he became James T. Kirk." That's been driving me nuts for years...there's other info here.