Almost immediately, the Royal Air Force (RAF) requested a number of B-17s. The USAAC agreed to divert twenty aircraft of an order for 38 new B-17Cs to England. The twenty B-17C Fortresses allocated to Britain under Lend-Lease were taken off the B-17C production line at Boeing. They were known as Model 299T by the manufacturer, and as Fortress I by the RAF. RAF serials AN-518 through AN-537 were assigned to these planes, although they were initially painted with the incorrect series letters AM rather than AN. The twenty Fortress Is were essentially similar to USAAC B-17Cs, but they had all but the single nose gun replaced by 0.5-inch Browning machine guns, and self-sealing fuel tanks were installed at Wright Field before the Fortress Is were flown to Britain.

Although the Army did not consider the B-17C as being combat ready (the E-version was already under procurement as the result of combat reports from Europe), the RAF was sufficiently desperate that these planes were immediately pressed into front-line service.

The Fortress I was to be used by No. 90 Squadron, based at West Raynham. Things got off to a bad start right from the beginning. On its delivery flight to Britain, the first Fortress I ran off the runway when landing at West Raynham, wiping off its undercarriage. It was never to fly again. It sat forlornly by the side of the runway, slowly being scavenged for spare parts.

The RAF planned to use the Fortress I on unescorted daylight bombing raids against targets in Europe, relying on the vaunted defensive firepower of the Fortress to fend off fighter attacks. The first sortie with Fortress Is was flown from Polebrook on July 8, 1941 against Wilhelmshaven. Three planes took part. Engine trouble forced one of the planes to divert to a second target, but the other two went on to attack the naval barracks at Wilhelmshaven from an altitude of 30,000 feet. Unfortunately, the planes were not able to hit anything from such extreme altitudes. In addition, their crews found that the temperatures at this altitude were so cold that their defensive machine guns froze up when they tried to fire them. However, all planes returned safely to base.

On July 24, a group of Fortresses attacked the French naval installation at Brest. They were equipped with the Sperry rather than the famous Norden bombsight. The Fortresses attacked from 30,000 feet and managed to miss their target completely. German fighters pounced on the formation, but all bombers returned to England. However, one of the raiding Fortresses was so badly shot up by the German fighters that it disintegrated upon landing.

A third Fortress I was lost in an accidental fire on the ground.

An operational sortie was made by three Fortresses against Oslo, Norway, but all three planes were destroyed by Luftwaffe fighters. One of the Fortresses apparently fell more or less intact into enemy hands, probably giving the Germans their first look at the American bombsight.

A seventh Fortress was lost during a high-altitude test flight. This plane was testing equipment at high altitude when something went wrong.

An eighth Fortress appeared in a vertical dive from out of a cloud and went straight into the ground. It was never determined what went wrong.

By September of 1941, RAF Fortresses had flown 22 attacks against targets such as Bremen, Brest, Emden, Kiel, Oslo, and Rotterdam. A total of 39 planes had been dispatched, out of which eighteen planes had aborted and two had been forced to bomb secondary targets because of mechanical problems. Eight Fortresses had been destroyed in combat or lost in accidents. Discouraged by these losses, the RAF decided to abandon daylight bombing raids over Europe.

Four Fortress Is were later sent to the Middle East, where, until May 1942, they undertook night attacks against enemy positions at Benghazi and Tobruk. In October of 1941, the few remaining Fortress Is in Europe were transferred to RAF Coastal Command, operating with Nos. 206 and 220 Squadrons.

One Fortress I found its way to India in July of 1942, and was taken over by the USAAF.

On the balance, the combat debut of the Fortress was a resounding failure. The Fortress Is encountered numerous mechanical failures during flight, their guns had an annoying tendency to freeze up at high altitude, and their defensive armament was completely inadequate to fend off determined enemy fighter attacks. In addition, it was found that it was much more difficult than expected to hit anything when trying to bomb from very high altitudes. As a result of RAF experience with the Fortress, it was determined that there was a need for vast improvements in defensive gunnery, a need for operating the Fortresses in greater numbers in tighter formations for better defensive firepower, and a need for better and more intensive crew training. Nevertheless, their British crews generally were quite pleased with the Fortress I, regarding it as easy to fly, very maneuverable, and aerodynamically stable in the bomb run.

Serial numbers of the Fortress I
USAAC	RAF
40-2043	AN-518
40-2044	AN-519
40-2051	AN-520
40-2052	AN-521
40-2053	AN-522
40-2055	AN-523
40-2056	AN-524
40-2057	AN-525
40-2060	AN-526
40-2061	AN-527
40-2064	AN-528
40-2065	AN-529
40-2066	AN-530
40-2068	AN-531
40-2069	AN-532
40-2071	AN-533
40-2073	AN-534
40-2075	AN-535
40-2076	AN-536
40-2079	AN-537