WWII

Dornier Do 17 – The “Fast” Bomber that was Too Slow

Today we are taking a look at one of Dornier’s most well known aircraft , the Do 17. Nicknamed the “Flying Pencil” after its long, thin fuselage, the Do 17 was originally built as a mail plane, but after one pilot from Luft Hansa noticed its excellent performance, it was soon on its way to being a combat aircraft.

In the period between the wars, Dornier Flugzeugwerke created some notable passenger aircraft. The vast and luxurious Do X flying boat was built to carry passengers across the Atlantic. The Komet and Merkur were small but sturdy passenger craft that saw extensive service with Luft Hansa, the German national airline.

The Do J Wal (Whale) was a flying boat that was used both as a small airliner and as a military maritime patrol aircraft. Dornier aircraft were generally reliable and functional, but there was little about them to set the pulses of aviation enthusiasts racing.

Then, in 1932, Dornier built something different, a sleek, fast mail plane that could also carry a handful of passengers. But Luft Hansa weren’t interested in a passenger aircraft that could carry only six people and the prototype was left to gather dust in a hanger at the Dornier works in Friedrichshafen.

Then, a pilot took it up for an impromptu test flight. The new aircraft handled like a fighter, though it was considerably faster than any contemporary fighter. Imagine if it could be converted to become a bomber…

Contents

Origin

In 1932, a specification was raised for the design of a new mail plane for Luft Hansa. Like most mail planes of the period, this was to be designed for high-speed and also to have limited passenger-carrying ability. Dornier responded with what was, for that company, a sleek and radical design. The Dornier Do 17 V1 was an all-metal, shoulder-wing monoplane with a single vertical tail fin.

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The cockpit was located at the front of the fuselage with two passenger seats immediately behind and four more in a separate compartment further down the slim fuselage. Power was provided by a pair of BMW V1 water-cooled V12 engines which gave the aircraft a very respectable top speed of over 200mph.

Three prototypes were delivered to Luft Hansa for testing. All were rejected – the cramped, split passenger accommodation was judged impractical for commercial operations.

Do 17-V1.
The awesome looking Do 17-V1.

The three prototypes were returned to Dornier and placed in storage. Then, after six months, one of them was flown by a Luft Hansa pilot, Flight Captain Untucht, who was stunned by the performance of the aircraft. It was as nimble as a fighter and extremely fast. He immediately suggested that it should be used as the basis for a fast bomber.

Unlike many other German civilian aircraft of the 1930s, it seems that the design of the Do 17 was not undertaken with potential future military use in mind. But by the time that the prototypes had flown, the Nazis had come to power and in 1935, the Luftwaffe was formally created. One of the concepts being explored by the new air force was that of the Schnellbomber (fast bomber). This focussed on the notion of bomber fast enough to outrun enemy fighters. With the Do 17 it seemed that Dornier might have the basis for just such an aircraft .

The Do 17

In late 1933, the Reichsluftfahrtministerium (RLM  – German Air Ministry) issued a specification for a “freight aircraft with special equipment.” German rearmament was still being concealed at this point, and special equipment was euphemism for a bomber. Work secretly began on converting the Do 17 mail plane into a fast bomber.

An internal bomb bay was added, the cockpit was enlarged to accommodate three crew members and the single vertical fin was replaced with a twin tail design to improve lateral stability. Defensive armament was light – the radio operator manned two 7.92 mm MG 15 machine guns in the rear cockpit while the bombardier operated a single MG 15 machine gun firing through the windscreen.

Do 17 nose.
Front nose section of the Do 17. This is an earlier version with the BMW VI inline V12 engines.

Power on production versions was provided by a pair of BMW in-line engines and it soon became apparent that the new bomber was fast. Very fast. In level flight, it was capable of well over 200 mph. But it was intended to bomb from a shallow dive during which it could exceed 300 mph. That made it significantly faster than any fighter then in service anywhere in the world.

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The new aircraft entered service with the Luftwaffe as the Do 17 E and the Do 17F (a reconnaissance version not fitted with a bomb bay). Development continued with several new models being tested with different engines until what would become the definitive version of this aircraft, the Do 17Z, was introduced following combat experience in the Spanish Civil War.

Do 17Z fast bomber.
A Do 17Z, the most produced version of the Do 17.

This was powered by a pair of Bramo Fafnir 323 radial engines and the crew compartment was enlarged to provide space for an additional gunner manning a ventral position housing a single MG 15 machine gun. Top speed in level flight was increased to over 250 mph and bomb load was increased to 1,000kg (on earlier versions, bomb load was just 500kg).

In Combat

During the Spanish Civil War, the German Condor Legion used the Do 17 extensively and it proved to be an effective bomber, easily outpacing Republican fighters, mainly Russian Polikarpov I-15 biplanes and I-16 monoplanes. This seemed to vindicate the Schnellbomber concept and by the time that German forces began the invasion of Belgium, Holland and France in the summer of 1940, the Luftwaffe had available over 400 of the early versions of the Do 17 and more than 300 of the improved Do 17Z.

However, it soon became apparent that the Do 17 had a fundamental flaw. While the performance of this aircraft had improved modestly with the introduction of the Do 17Z, the performance of fighters had made rapid progress. When this aircraft was first introduced in 1935, the front line RAF fighter was the Hawker Fury biplane which struggled to reach 200mph. By 1940, the RAF was equipped with the Supermarine Spitfire with a maximum speed of over 350mph.

Hawker Fury biplane.
At the time of its introduction, the Do 17’s competition was aircraft such as this Hawker Fury biplane fighter.

The Do 17 was designed from the beginning as a fast bomber that would outrun enemy fighters. To achieve maximum performance, it was provided with little armour and only light defensive armament.

The Do 17 hadn’t got any worse, but in the years between its introduction and its use on the western front in 1940, the fighters it would face had become exponentially better. The fragile Do 17 was designed to survive by outperforming enemy fighters, but by 1940, it was no longer capable of doing that.

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During the Battle of Britain in the late summer and autumn of 1940, the Do 17 would prove to be the most vulnerable of all the Luftwaffe bombers (it was also during this battle that the Do 17 gained the RAF nickname by which it is now remembered, the Flying Pencil). Around 220 Do 17s were used in this campaign, mainly the improved Do 17Z version. Despite its speed and nimble handling, in August alone, 74 were lost to combat damage and accidents and in September, 50 more were written-off. In all, more than half the Do 17s used by the Luftwaffe were lost during the Battle of Britain.

Do 17s above London.
Do 17s during a bombing raid over London.

In a desperate attempt to improve its survivability, additional defensive machine guns were added in the field. During the battle, some Do 17s went into action carrying up to eight MG15s, but this didn’t slow the alarming rate of attrition. By the time that the Battle of Britain ended, the Do 17s deficiencies had been recognized and all production ended in the summer of 1940.

The Do 17 would continue be used during German actions in the Balkans and Greece and in the early part of the German invasion of Russia that began in the summer of 1941, but it was increasingly being replaced by newer and more effective bombers including the Junkers Ju 88 and an improved version from Dornier, the Do 217, that was more robust and able to carry a heavier bomb load.

Do 17 cockpit interior.
Interior view of the Do 17’s greenhouse nose and cockpit.

From 1940, a few Do 17s were converted for use as night-fighters. On what became known as the “Kauz” the glazed nose was replaced with a solid version housing three fixed 7.92 mm MG17 machine guns and a single 20mm MG FF cannon. This version was notable for the provision of the Spanner-Anlage (roughly Peeping Tom) IR detection system. This consisted of an IR scope for the pilot and an IR searchlight mounted under the forward fuselage and used to illuminate targets.

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This was, arguably, the very first FLIR system fitted to any combat aircraft, but it was also almost completely useless. Pilots noted that it wasn’t possible to detect a target at beyond 200 metres, at which point its exhausts were visible anyway even in darkness. By early 1941, most Do 17 night fighters had been replaced by more effective modified Ju 88s and Bf 110s.

Do 17 night fighter.
A Do 17Z-10 equipped with Lichtenstein radar.

In Luftwaffe service, the Do 17 ended its military career rather ignominiously by being redeployed as a glider tug, with some being used to provide supplies to beleaguered German forces in Russia. This aircraft also served in the air forces of several German allies during World War Two including Finland, Bulgaria, Romania and Yugoslavia, but few of these were used in combat operations.

Conclusion

The Schnellbomber concept seemed to make sense in the mid-1930s, when it was possible to build fast, all metal, monoplane bombers that were considerably faster than the generally biplane fighters then in service. But the concept had an inherent flaw: to give the required speed, crew protection and defensive armament were sacrificed. Speed gave the bomber protection, but only if it was faster than the fighters it faced.

Perhaps it should have been obvious to the Luftwaffe and to Dornier that this would only be a viable concept until the new generation of high-speed monoplane fighters then in development entered service. On paper, in the late 1930s, the Do 17 seemed to be uncatchable. In the skies over southern England in 1940, it proved relatively easy prey for the new Spitfires and Hurricanes of the RAF.

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The Do 17 was a narrow-focus design that sacrificed everything for speed. When that speed proved insufficient, the Do 17 was withdrawn from service and replaced by most robust bombers that emphasized survivability and defensive armament as much as speed. That’s the problem with any “one-trick pony” – when the trick is negated, all you’re left with is an ordinary pony. Or in this case, a Schnellbomber that just wasn’t fast enough to survive in combat.