some very thorough and technically correct responses here, but I found the way they were presented to be a bit confusing.
WHAT YOU NEED TO KNOW
26" as was the common size on mountain bikes for many years is NOT the same as 650B. 650B rim has a bead seat diameter (the dimension that mates with the tire) of 584mm, while a 26" mountain bike wheel has a bead seat diameter of 559mm. A 650B tire will be too big to fit on a 26" rim.
The Sheldon Brown article linked above is the best resource if you want to learn about this.