Block B formed part of the Government Code and Cipher School at Bletchley Park and together with Block A (Monument HOB UID 1525270) was the first purpose built brick building for office accommodation on the site. Construction was completed in August 1942 and it originally housed the Naval Section dealing with non-Enigma cryptography in Western Europe, together with associated Research and Crib Sections, Japanese Cryptography and part of Air Section which included the German call-sign Research Section. By mid 1943 the Naval Section had taken over all the building and the Air Section had moved to Block F (Monument HOB UID 1525325).
Block B is a two storey, steel-framed building with pre-cast concrete floors and roofs, and brick walls. The building is attached to Block A to the west and has a dog-leg plan, with spurs to south, to the north, and to the east of the main entrance.
After departure of the Government Code and Cipher School in 1946 Block B, together with Block A , was used as a National Service Hostel by the Ministry of Labour. In 1949 it was part of the Emergency Teacher Training College which had been set up in the park. Alterations to the building took place throughout the 1950s. In 1977 the building had been taken over by the Civil Aviation Authority (CAA) mainly for office use. Following the departure of the CAA from the site in 1993 the building became the responsibility of Bletchley Park Trust and in 2004 was refurbished as new exhibition space for the Bletchley Park National Codes Centre Museum.