TexC, Farmall B first produced in 1939, same time as the Letter Series A, H, and M. The B has adjustable rear wheel tread from 64 to 92 inches, in 4 inch intervals. The B will cultivate two rows spaced from 32 to 46 inches.
Farmall BN first produce in 1940. It has been said it was first produce in response to farmers who wanted a little narrower tread for certain crops. The BN has adjustable rear wheel tread from 56 to 84 inches, in 4 inch intervals. The BN will cultivate two rows spaced from 28 to 42 inches.
Both B and BN are either single or dual front wheel tricycle tractors. Both B and BN can be fitted to cultivate up to 6 rows of narrow row crops. Both Band BN are "Culti-Vision" tractors. Engine and transmissions are basically identical to the Farmall A's. Engine is basically identical to the Farmall C engine. Both are handy little tractors for utility or mower work as well as cultivation.
It is hard to say exactly how many of each were made as, production numbers are blurred in with the Farmall A. Most agree less BN's were made, making them somewhat more dear than B's. Both B and BN were replaced after 1947 by the Farmall C.