By 1943, it was becoming apparent that the standard propeller-driven piston-engine aircraft was reaching the limits of its potential speed. At the same time, though, Nazi Germany was facing daily raids by American and British bombers, and needed ever-faster planes with ever-higher altitudes to fight back. In the race to increase speed and power, unconventional designs were looked at, and one of the oddest was the Dornier Do335 "Pfeil" fighter, which had two propellers--one mounted in front and one at the back.
The Dornier Do 335 A "Pfeil" fighter, on display at the Smithsonian's Udvar-Hazy Center
By the later stages of World War II, aircraft designers on all sides were frantically looking for ways to increase the speed, altitude and power of their planes. The ultimate goal was jet power, but the early jet engines of the time were unreliable and required a huge amount of maintenance work. The alternative was to refine the standard propeller-driven piston engine to produce more speed and power. The fastest regular-production piston airplane of the time was the De Haviland DH98 Mosquito, which used two engines and a lightweight wooden fuselage to reach speeds of over 400 mph. But the Mosquito did not have the power to carry a sufficient weight of armor and weaponry at the speeds and altitudes necessary to intercept high-flying bombers. A new design was needed.
There was a logical step to be taken. One of the limits on the Mosquito's speed was the drag produced by its two engine nacelles, one on each wing. If these two engines could somehow be placed inline, one directly behind the other, it would reduce the drag without reducing the power, thereby increasing speed. Such an idea had already been tried before--during the First World War, the experimental Fokker M-9 in 1915 had used a two-engine configuration, with one engine mounted at the front of the fuselage and a second "pusher" engine mounted at the rear. Dornier later used a similar configuration in some seaplanes in the 1920's. The concept of having two inline engines in the fuselage was known as "centerline-thrust".
In the United States, designers at the Republic company reduced this idea down to its most minimal basics--in the XP-72 "Superbolt", instead of using two separate engines fore and aft, they took a single much-larger engine at the front of a P-47 Thunderbolt airframe and put two propellers on it that were counter-rotating--each spinning in a different direction. This eliminated drag, saved weight, and the counter-rotation prevented torque which would have affected the plane's handling. The new plane had an estimated speed of 550mph--almost as fast as a jet engine. Although a handful of prototype Superbolts were built, the war ended before they went into production.
The German idea was less radical but was based on the same centerline-thrust idea. The Dornier Do335 Pfeil ("Arrow") was designed with two engines and two propellers, one fore and one aft. The aft engine was located just behind the cockpit to keep the airplane balanced, and used a long shaft to turn the rear propeller. The two-engine design allowed for a much slimmer fuselage than the single-engine Superbolt design, with less drag and more aerodynamic efficiency. The engines were Daimler-Benz 12-cylinder inlines, producing a top speed of over 470mph--far faster than any existing Allied fighters, and powerful enough to intercept even the new American B-29 high-altitude bomber. The fighter version of the Pfeil would be armed with two 15mm cannons on the nose and a single 30mm cannon firing through the front propeller hub. One 30mm shell was capable of destroying a four-engine bomber.
Because its speed made bailing out of a damaged Pfeil in a parachute impossible, the plane was equipped with one of the first ejection seats, which used explosive bolts to remove the canopy, the tail fin and the rear propeller, and then a small air piston to throw the pilot's seat clear of the plane before deploying a parachute.
Originally, the Pfeil was designed as a super-fast bomber with a 1,000 pound bomb load. But as the war turned against Germany and British Lancasters and American B-17s began pounding Nazi cities, the plane was redesigned as a multi-role fighter/interceptor, reconnaissance, and (in a two-seat version) night fighter. The first test flights began in the winter of 1943. Twenty-three protoypes and test versions were built. Production of single-seat A-1 combat versions began in late 1944, but were delayed by supply problems and ended when advancing Allied troops captured the factory at Oberpfaffenhoffen in April 1945. Only eleven combat-version Pfeils (and two twin-seat trainers) had been delivered, and these were used by the Luftwaffe as reconnaissance planes. None ever saw combat.
When the Allies overran the production plant at Oberpfaffenhoffen, one of the pre-production prototype Pfeils, serial number 240102, was still sitting on the grass outside. It and a second Pfeil were flown to France and then shipped on a British aircraft carrier to Maryland for testing by the US Navy. The fate of the second Pfeil is not known, but for three decades after its flight testing, Number 240102 sat outside at the Navy base in Norfolk, Virginia, despite having its ownership transferred to the Smithsonian Air and Space Museum in 1961. In 1974, the deteriorating Pfeil was the only one of its kind remaining, and the Smithsonian shipped it to the Dornier factory in West Germany for a complete restoration, after which it was loaned to the air museum in Munich. In 1986, the Pfeil was shipped back to the US for storage at the Paul Garber facility. Today, Pfeil Number 240102 is on display in the Smithsonian's Udvar-Hazy facility.