Boeing B-17s Fortress Mk.I and Mk.IIs in RAF Service

July 8, 1941: 90 Sqn debuted the Fortress Mk I in combat.

July 1942: The Boeing Fortress II entered service with 220 and 206 Sqns at Benbecula

Some 45 B-17E bombers were transferred to the RAF in late 1942 for use by Coastal Command designated as Fortress Mk IIA (the designation Fortress Mk I had been used for 20 B-17C aircraft) and, with the newer B-17F that had been delivered to the RAF earlier than the B-17E thus receiving the designation Fortress Mk II, served with four maritime and four meteorological reconnaissance squadrons.

The Fortress Mk. IIA was the British equivalent of the B-17E, as shown here by 41-9141 wearing a British-style camouflage finish via American manufactured paints, although this particular aircraft spent much of its time in the US

Before the British-built four-engined bombers became available, the government had purchased a quantity of Boeing B-17C aircraft which they christened Fortress I. These arrived in England on 14 April 1941 and were issued to No. 90 Squadron, who used them operationally for the first time on 8 July against Wilhelmshaven.

It is often forgotten that it was Britain’s RAF, and not the American services, which first flew the B-17 in combat, this taking place during July 1941.

The Americans had strongly urged that the aircraft should not be used operationally as they were still suffering from teething troubles, but this advice was ignored and they were introduced in an attempt to provide a high-altitude bombing force. During the next three months No. 90 Squadron encountered problem after problem, losing several aircraft to enemy fighters and unexplained crashes, which eventually led to the Fortress I being withdrawn from European operations by the RAF in September.

Coastal Command

By September 1941, eight of the original Fortresses Mk.Is had been lost through various causes. With newer and more reliable heavy bombers coming into service by October 1941, the remaining aircraft were absorbed by 220 Sqn, RAF Coastal Command, and based at RAF Wick in northern Scotland to be used as long-range maritime patrol aircraft. In July 1942, by which time 220 Sqn was based at RAF Ballykelly, Northern Ireland, it had received the Fortress Mk II, as did 206 Sqn based at RAF Benbecula in the Outer Hebrides; in December 1942, 59 Sqn based at RAF Thorney Island in Sussex, would also receive the Fortress II.

Starting in mid-1942 about 150 of the improved B-I7E were delivered to Coastal Command as the Fortress Mk II and Fortress Mk IIA, serving with Nos 59, 86, 206 and 220 Squadrons, operating from Benbecula, Chivenor, Thorney Island, the Azores and lceland, Although possessing shorter range than the B-24 Liberator, the Fortress contributed considerably to the patrol efforts demanded by the frequent sailing of wartime convoys, particularly at the height of the great U-boat campaign. ln the Atlantic, RAF Fortresses were employed on anti-shipping strike missions, their weapons being almost entirely confined to depth charges,

The Flying Fortress has generally been overlooked due to the successes of many other types involved in the war against the U-boats, such as the Sunderland, Liberator and even the Wellington. Nevertheless, it had more success than when it had briefly served with Bomber Command and, because of that, the significant part that it and its crews played in the Battle of the Atlantic should not be forgotten.

Testing the Fortress

Fortress Mark I

The promise of the well defended Boeing B-17 led initially to a small order for the -C model for the RAF; the first to reach the Boscombe Establishment, AN53 I. was delivered on 15 April 1941. The Fortress Mk I created a very favourable impression for its ease of handling. even with two engines at idle but not feathered, and particularly for its comfort.

Small stick movement by the pilot operated tabs on the elevator or rudder, but larger movements fed directly to these control surfaces: this feature together with an appropriate stability made handling easy and precise. Electric motors for flaps and undercarriage (40 secs to raise) were novel. The manual rudder trim had a wide operating range and, fully applied at 120 mph (indicated) enabled straight flight to be maintained with the two starboard engines alone at maximum cruising power without the use of the rudder pedal. Most praised was the warmth of the crew area at 30,000 ft (-55°C outside), and the relative quiet achieved with copious soundproofing combined with favourable positioning of the engine exhausts. The navigator’s position was roomy, with a good view; the only criticism of this first version of the type was the lack of blackout curtains. Performance with the Cyclone R-1820-73 was outstanding at height with a ceiling of 34,000 ft from a maximum weight (49,300 Ib) take-off, although great care had to be taken to avoid damage above 25,000 ft by overspeeding of the exhaust-driven turbocharger. Crews were carefully briefed on the special handling needed. From August 1941, A 519 also handled well at extended aft CG, but, even with by-pass flame dampers of RAE design, did not meet the requirements for invisibility at night from a distance greater than 100 yards. Armament trials were limited to assessment of the American bomb gear (the maximum normal load was four 600 Ib) and then, at the end of 1941, partially successful attempts to fit British carriers; restricted space in the bay rendered the usual bomb hoist, made by Stones, useless. A most surprising omission was the apparent absence of any trials on the five single handheld guns: operationally the defensive armament of this early version proved inadequate.

Fortress Mark II and IIA

Improvements in the next version. the B-17E or Fortress IIA were soon under scrutiny following arrival of the first, FK 187, in April 1942. A collapsed tail wheel (necessitating replacement by FL458) in August) and the need for modifications to the gun positions delayed completion until early 1943. The electro-hydraulic Sperry upper turret (two 0.5 in guns) was smooth and positive in operation up to 300 mph (indicated); some 9,700 rounds were fired. The two beam and twin tail guns were satisfactory, but the Sperry ball turret under the fuselage had a very poor view, was cramped and awkward to use, and suffered many early failures of the ammunition feed. Later in May and June 1943, FK211 with modified feed arrangements to the ball turret was assessed as satisfactory. Reports on bombing trials included ground assessments of the bomb aimer’s position, and checking of the various loads which could be carried. Seven flying hours with the Norden auto-flight system demonstrated its ability to control turns at all speeds and to hold heading even with two engines on one side fully throttled-the resulting sideslip was, however, uncomfortable. Following performance checks on FK 187 the Establishment extrapolated the figures and assumed a full load of 1,440 imperial gallons of fuel (plus 115 gal of oil) to give a take-off weight of 51,350 lb. With standard allowances, an average of 1.42 miles per gallon gave a theoretical range of 2,020 miles (1,620 miles for practical planning). The Establishment further extrapolated these figures assuming 600 gal in the bomb bay. and calculated a maximum still air range of 2,890 miles (2,360 practical); the exercise was to help the Air Staff to assess the type’s effectiveness for the maritime role, All Fortress IIs were used in this way by the RAF.

Flame damping was good on the Fortress IIA. and the navigational facilities adequate. including the astrodome in FA706 (a B-17F, Fortress II) in January 1943. Rocket projectiles, planned for the Fortress but never fitted, led to preliminary attitude measurements on FK211 early in 1943. A 40 mm Vickers gun was however, installed on FK 185 and tested from December 1942; about 700 rounds were successfully fired. A feature of the complementary handling trials was the increase in all up weight to 52,000 Ib: previous Boscombe flying had been limited to under 49,400 lb. The High-Altitude Flight used FK 192 for eighteen months from June 1943 on meteorological research.

The need for measurement of humidity was soon identified. and two types of hygrometer (modified from sea level types) were sent by distinguished scientists. but proved impractical. George Hislop, who, by late 1941 had a meteorological assistant with independent views, developed a nephelometer, testing it in a Fortress, thus becoming the first to measure humidity at altitude. Among discoveries was relative humidity of 105% – due, it was agreed, not to false readings. but to supercooled water droplets.

Fortress Mark III

The special electronic role of some RAF Fortress III (B-17G) was cleared for Service use by brief handling trials on HB767 at RAF Oulton: the lower than expected stalling speed was attributed to the pressure error caused by the H2S radar fairing. Further trials, also in July 1944. on HB774 established that the errors were. in fact. small. The Establishment used three Fortress III at Boscombe: H B702 briefly from November 1944 for gunnery, and K L835 from July 1945 to establish civil operating criteria.


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