The attempt on his life in November 1939 may have been staged, or rather, it was known beforehand. The assassin, Georg Elser, may have been an unwilling dupe of the Gestapo, who had learned of his desire to kill Hitler, when Elser was imprisoned in Dachau in the late 30s.
From records captured at the end of the war, it looks like the Gestapo used Elser in their larger effort to smoke out men in the Abwehr who were suspected of communicating with the British, through contacts in the then-neutral Netherlands.
There are records that show that Elser was given his release from arrest, and that materials to make the bomb were supplied to him.
He was a trade unionist and Communist, a cabinet-maker by trade and a tinkerer, but by most accounts, not too bright, so the Gestapo considered him the perfect tool.
It is not documented explicitly whether Hitler himself knew about the bomb, but it was noted that he and the other top party officials often stayed long after the speeches were done, to reminisce about the old days-the occasion was the anniversary of the Beer Hall Putsch. But on that night, his speech ended unusually early, and the bigwigs also left.
Two British officers who had been in contact with German officers in the Netherlands were kidnapped there and spirited off to Germany, where they were accused of abetting the plot. If I am not mistaken, they were murdered in prison.
Elser was arrested, tried and convicted, but remarkably, kept alive until April 1945, when he was murdered on the orders of the Gestapo.
William L. Shirer gives an account of that particular plot in The Rise and Fall of the Third Reich; he found the details during his research into the mounds of paper left behind when the Reich collapsed.