Volvo entered the tractor market by teaming up with Swedish tractor manufacturer Bolinder-Munktell in 1943. The tractors from both firms were similar, and Volvo acquired Bolinder-Munktell in 1950. The separate brands were kept until 1957, when all tractors were produced under a joint Bolinder-Munktell-Volvo brand. The brand was changed, again, in 1973 to Volvo BM because of the greater recognition of the Volvo name. The company entered into a joint venture with Valmet in 1978, and sold all tractor interests to Valmet (now Valtra) in 1985.
Valtra - official site
Volvo tractors - information from Volvo
BM Volvo tractors - site dedicated to BM Volvo tractors