The origins of the Do 335 trace back to World War I when Claude Dornier designed a number of flying boats featuring tandem engines. The same system was also used on the very successful post-war Wal and Do 18. In a tandem layout the engines are mounted back-to-back in pairs, the front engine 'pulling' and the rear one 'pushing'. There are many advantages to this design over the more traditional system of placing one engine on each wing. You have a twin-engine layout with the frontal area (and thus drag) of a single engine design, allowing for higher speeds. It also keeps the weight near the centerline, so the plane can roll faster than a traditional twin. In addition an engine failure doesn't lead to asymmetric thrust, and even in normal flight there is no net torque so the plane is easy to handle.

In 1939 Dornier was busy working on the P.59 high speed bomber project, which featured the tandem engine layout. Work on the P.59 was stopped in early 1940 when Hermann G?ring ordered the cancellation of all work which would not be complete within a year or so.

In May 1942 Dornier submitted an updated version with a 1,000kg bombload as the P.231, in response to a requirement for a single seat high speed bomber/intruder (other entries included the Messerschmitt Me 155). P.231 was selected as the winner after beating rival designs from Arado and Junkers, and a development contract was awarded as the Do 335. In the Autumn of 1942 Dornier was told that the Do 335 was no longer required, and instead a multi-role fighter based on the same general layout would be accepted. This delayed the prototype delivery as it was modified for the new role.

Fitted with Daimler-Benz DB 603A engines delivering 1,750hp at take-off, the first prototype flew in October 1943. The pilots were surprised at the speed, acceleration, turning circle and general handling of the type -– it was a twin that flew like a single. The only sore spots they found were the poor rearward visibility and weak landing gear. V2 and V3 incorporated several minor changes; the oil cooler under the nose incorporated into the annular engine cowling, blisters were added to the canopy with small rear view mirrors, and the main undercarriage doors were redesigned.

On May 23, 1944 Hitler ordered maximum priority to be given to Do 335 production. The main production line was intended to be at Manzel, but a bombing raid in March destroyed the tooling and forced Dornier to set up a new line at Oberpfaffenhofen. The decision was made to cancel the Heinkel He 219 and use its production facilities for the Do 335 as well. However, Ernst Heinkel managed to delay, and eventually ignore, its implementation.

The first ten Do 335A-0's were delivered for testing in May. By late 1944 the Do 335A-1 was on the production line. This was similar to the A-0 but with the uprated DB 603E-1 engines and two underwing hard points for additional bombs or drop tanks. Capable of a maximum speed of 474mph at 21,325ft with MW 50 boost, or 426mph without boost, and able to climb to 26,250ft in under 15 minutes, the Do 335A-1 could easily outrun any Allied fighters it encountered. Even with one engine out it could reach about 350mph, allowing it to escape combat fairly easily.

Delivery commenced in January 1945. When the US Army overran the Oberpfaffenhofen factory in late April 1945, only eleven Do 335A-1 single seat fighter-bombers and two Do 335A-12 conversion trainers had been completed