The heartland of Bantu languages is somewhere in the Congo River basin rain forest. This is supported by evidence from comparative linguistics which shows that the farther one goes from this assumed heartland, the bigger difference between the so-called Bantu languages tends to become. However as regards the origin of Bantu people and their languages, though nothing is a hundred percent certain about this issue because of a long obscure past (± 2 millenniums BCE) and scarcity of written records, there is enough evidence (such as archeological evidence and oral accounts) which can allow us to hypothesize that Bantu people moved from some other places to settle wherever they are found today.
Their migrations occurred in gradual waves starting in mid-second millennium BCE (or probably much earlier than that) and finally ending before 1500 CE. Many authors believe Bantu people originated from southern West Africa in the region between the Niger delta and the Sanaga River in Southern Cameroon. From there, they moved eastwards and southwards as summarized by Cartwright on the following map.
 By ‘heartland’ I mean the part of a region considered essential to the viability and survival of the whole, especially a central land area relatively invulnerable to change or attack.
N.B. This map shows the boundaries of present-day African nations in the background so as to facilitate situating Bantu movements more easily with reference to modern administrative division of Africa.