Months before the USA started its participation in WW2, the B-17 four-engine bomber had seen combat … and had been shot down.

RAF’s Bomber Command acquired 20 B-17C ‘Fortress I’ (photo) from the US in late 1940, flying their first combat mission on 8 July 1941, (6 months before Pearl Harbor), when three B-17C departed RAF Watton to bomb the German naval base at Wilhelmshaven from high altitude (30,000 ft) in broad daylight.
The Germans fighters failed to intercept the high-flying bombers, but the engine of one of them failed and it diverted to a secondary target. The other two bombers proceeded to the German port but at the high altitude they were flying the low temperature congealed the machine guns’ lubricant rendering them non-operational. Bombing is reported as ‘ineffective’.

By September, eight B-17Cs had been lost in operations (combat and accidents). Bomber Command retired the bomber due to numerous mechanical breakdowns and its vulnerability to fighters caused by light defensive armament and light armor. Relatively small bomb loads and low aiming accuracy are sometimes reported -it seems the B-17C did not employ the Norden bombsight-. The remaining bombers were handed out to Coastal Command for anti-submarine patrols.

The Luftwaffe claimed to have shot down nine RAF B-17s but three of the claims precede the Fortress debut, so they are probably misidentified.
The first B-17 shot down can probably be attributed to Uffz. Karl Pfeiffer, flying a Bf 109E-7 or F (1 or 2) from 3./JG 2 over La Pallice in France on 23 July 1941 at sunset. Another pilot, Georg Schmalenberg from the same unit, claimed a second B-17 minutes later.
The B-17C (see picture) lacked the machine gun turrets later installed in the B-17E. It had one MG in the dorsal, nose, and ventral (bathtub) positions and one each in the port and starboard side blisters. Its empty weight of 30,900 pounds was 1351 pounds lighter than the B-17E, the model initially used by the USAAF in Europe.

The B-17E, the first heavily armed Fortress, that the USAAF employed in Europe, introduced three twin-gun turrets to increase defensive firepower: tail, ball, and top.