True stories of humans rescued and raised by wolves exist. For example, in the nineteenth century, Dina Sanichar was discovered at age six living in a cave among wolves in India. But while Rudyard Kipling may well have had in mind Sanichar's story when he invented Mowgli, The Jungle Book is not a true story.
Sanichar might have been an inspiration, but the wise and wily Mowgli is far different from the real-life mentally impaired Sanichar. Further, the stories in The Jungle Book are fantastic in that animals in real life can't speak and have not, at least to human knowledge, developed the kind of cognitively sophisticated culture that Kipling depicts among them.
Rather than trying to capture the factual reality of the life of a feral child, Kipling uses this backdrop to explore themes or truths about life in general. For example, he examines the importance of order to any society. The Law of the Jungle looms large in the young Mowgli's life, showing the importance of restraints and boundaries on individuals to ensure the survival of the group. Shere Khan, for example, is a "fool" and enemy to most of the animals, not because he is a predatory tiger but because he disregards the rules of the jungle. For instance, he hunts cattle, risking bringing the wrath of the humans on the animals.
While The Jungle Book as a whole might be inspired by true events and contain universal truths, it is otherwise a work of fiction.