Well, that took more looking than I expected. Would have sworn it was in:
Robin Neillands "The Bomber War"
And it may be, though I couldn't find it there.
Lately have seen it mentioned on one of those History Channel shows, but know I first read it somewhere years ago, but where....?
Ah, here it is:
Frederick Taylor, Dresden, page 117
"In 1940 the Germans had a very underdeveloped air defense, with only forty defensive fighters at its disposal. By the late summer of 1941 the system had been dramatically augmented, especially around key areas such as the Ruhr. In the first eighteen nights of August 1941, some 107 British aircraft were lost; in September, a total of 138 (62 crashed inside England); and in October 108. A shocking 12.5 percent of those sent to Berlin, 13 percent of those sent to Mannheim, and 21 percent of those bombing the Ruhr did not come home.
"Added to these figures was the implication, from the Butt report, that these raids were not just costly in money, men and aircraft, but near enough futile. They just weren't hitting much. On November 13, 1941, Bomber Command was ordered to halt long-range operations. This was, for the moment, as serious admission of failure.
"However, with no progress on any land front, and the public still shocked and embittered by the effects of the Blitz, there was no question that, in the longer term, bombing Germany -- "hitting back" -- remained a key component in the machinery of morale maintenance.
"As a result, whatever the concern felt in government circles about the effectiveness of Bomber Command's raids, this was not the message put out by the Ministry of Information. Newsreels and skillfully made documentary films such as Target for Tonight (released in August 1941) had served to convince most British civilians, and many Americans also, that the RAF was doing an excellent job over Germany.
"They did not know that so far more RAF aircrew had been lost over Germany than enemy civilians killed on the ground."
British area bombing resumed on March 28, 1942.