B-tree means "binary tree" and are not inherently balanced. There are plenty of specific types of self balancing tree algorithms, but I believe the default implementation of b-trees by definition are not self-balanced.
Edit: this comment is incorrect. There is a difference between b-trees and binary trees.
After a talk at CPM 2013 (24th Annual Symposium on Combinatorial Pattern Matching, Bad Herrenalb, Germany, June 17–19, 2013), Ed McCreight answered a question on B-tree's name by Martin Farach-Colton saying: "Bayer and I were in a lunch time where we get to think a name. And we were, so, B, we were thinking… B is, you know… We were working for Boeing at the time, we couldn't use the name without talking to lawyers. So, there is a B. It has to do with balance, another B. Bayer was the senior author, who did have several years older than I am and had many more publications than I did. So there is another B. And so, at the lunch table we never did resolve whether there was one of those that made more sense than the rest. What really lives to say is: the more you think about what the B in B-trees means, the better you understand B-trees."[3]
"The B-tree is a generalization of a binary search tree in that a node can have more than two children (Comer 1979, p. 123). Unlike self-balancing binary search trees, the B-tree is optimized for systems that read and write large blocks of data."
Long ago in one of my CS classes, I wrote binary tree, 2-3, 2-3-4, red black, and AVL tree algorithms. So I guess I learned about binary vs self-balancing. I did not know about b-trees. I made the mistake of thinking that b-tree meant binary tree.