In the January 1993 interview with game's programmer Yuji Naka published in the Beep! magazine, he stated that the rumors about Tails being a girl or Sonic's girlfriend wasn't true, he was always intended to be a boy from the very beginning. He also stated that Tails' name was originally Miles for most of the game's development. His nickname first came up from during a meeting with a producer from ABC.
Contributed by ProtoSnake