The BBC is to celebrate 40 years of Dad's Army with two on-air specials and the release of several documents and images charting the series' early development.

The sitcom first aired on BBC One on July 31, 1968, became an instant hit and regularly achieved audiences in excess of 15m over its 80 episode run.

Among the documents released by the BBC Archive are a series of heated memos exchanged between Michael Mills, then the BBC's head of comedy and light entertainment for television, and BBC One controller Paul Fox regarding topics ranging from the series' very existence down to its now famous animated arrows title sequence.

In a May 1968 memo, Mills told Fox of his "profound disquiet" over the latter's decision to remove shots of refugees and Nazi troops from the series' titles. "I cannot help wondering whether we, in the Comedy Department, are controlled by different standards, i.e. clowns must stay clowns," he wrote.

Fox's response questioned the show's existence: "After what I have seen so far, I think one must be allowed to wonder whether Dad's Army does indeed advance 'Comedy's output into new areas'. Is this really breakthrough territory?"

A subsequent memo from Fox dated 1970 offered Mills and co-creator David Croft "many congratulations and grateful thanks" for the show's success.

On television, Jonathan Ross will host a tribute show on BBC One at 7pm on Sunday, August 3; the previous night, BBC Two plans to screen an evening of classic episodes.