From Wiki;
Of the 1.5 million tons of bombs dropped on Germany by U.S. aircraft, 640,000 tons were dropped from B-17s.
[8]
On 30 May 1942, between 0047 and 0225 hours, in
Operation Millennium 1,046 bombers dropped over 2,000 tons of high explosive and incendiaries on the medieval town of
Cologne, and the resulting fires burned it from end to end. The devastation was nearly total. The fires could be seen 600 miles away at an altitude of 20,000 feet. Some 3,300 houses were destroyed, and 10,000 were damaged. 12,000 separate fires raged destroying 36 factories, damaging 270 more, and leaving 45,000 people with nowhere to live or to work. Only 384 civilians and 85 soldiers were killed, but thousands evacuated the city. Bomber Command lost 40 bombers.
From the USAAF Strategic Bombing Survey, by way of Ray Wagner on page 133, table 7;
B-17 MISSIONS = 291,508, BOMBS DROPPED = 640,036 TONS.
That is 4,391 pounds per mission, or about 56% per mission of the Lancaster's average!