During the First World War Prof. Claude Dornier designed a number of flying boats, which typically featured a tandem engine installation. The engines were mounted back-to-back in pairs, with the forward unit driving a tractor airscrew and the aft facing unit a pusher propeller. This engine arrangement was subsequently adopted for the highly successful Dornier Do Wal flying boat of 1922.
The idea of placing the pilot between the two engines in such an arrangement obviously occurred to Dornier. Indeed, on 3rd August 1937 he filed patent number 728044 for an aircraft of just such a configuration. It was on the basis of this patent that the Do 335 came to be developed.
An extension drive shaft from the rear engine was introduced for the first time in the Göttingen Gö 9, a 1 : 2,5 scaled down Do 17, build by Schempp-Hirth in Nabern completly fom wood. The extension drive shaft could been tested sucsessfully.
Fitted with Daimler-Benz DB603A-2 engines delivering 1750 hp at take-off, the first example, Do 335 V1 (CP+UA), flew for the first time on 26 October 1943 from Mengen, Wurttemburg, with Flugkapitan Hans Dieterle at the controls.
After initial handling trials at Oberpfaffenhofen, the Do 335 V1 was ferried to the Rechlin Erprobungstelle for official evaluation. Although some snaking and porpoising was found at high speeds, the Rechlin test pilots were generally enthusiastic. They commented favourably on its general handling behaviour, manoeuvrability and in particular on its acceleration and turning circle. However, they also criticised the very poor rearward vision and weak undercarriage.
Top speed was 775 km/h, which made it the fastest piston-engine plane of the war.
The Do 335 V4 was intended to be the prototype for the two-seat Do 435 night and all-weather interceptor, featuring side-by-side seating, cabin pressurisation, 2500 hp Jumo 222 engines and long span wooden outer wing panels.
All in all 32 planes were finished until the end of the war, 3 more unter controll of the allies after the war.
One plane survied in open storage for 27 years in the grounds of the NASM storage facility at Silver Hill. In October 1974 the decaying airframe was flown back to Munich, for a complete restoration by Dornier Aircraft at Oberpfaffenhofen. The magnificently restored aircraft was displayed at the Deutches Museum, Munich, before returning to the NASM, where the planes is unfortunatly not displayed.
I highly recomment the book
"Vom Original zum Modell" by K. H. Regnat.
  maiden flight  Transport of the fuselage to the maiden flight of the Do 335 V1 (WerkNr. 2300), that took place at Mengen/Württ. on 26. 10.1943.
The registratin number was CP+UA. You can see the rearengine DB 603E and the extension drive shaft
  V1  The complete frontengine could be replaced. Again the V1.
  V1  The V1 had round wheelcovers.
  V1 im Flug  The V1 in flight.
  V11  The V11, a doubleseater, unarmed trainerversion.
  V17  The V17 (Werknr. 240 313), a doubleseater Do 335 B-6 nightfighterversion after a landingcrash, 27.11.1947 at Bretigny (France). This plane was finished after the war, had its maiden flight on 2.4.1947 and was then flown to France.
  V17  A second cockpit for the radar operator was inserted behind the normal cockpit.
  V17  The wrecked V17 (Werknr. 240 313) in Bretigny.
  V11  The B-series cockpit had an armoured front screen. This is the V14.
  V11  The V14 was a B-2 destroyerversion with two MK 103 in the wings. It was tested in 1945 in France.
  Cherbourg  In July 1945 the Werknummer 240101 und 240102 were put aboardin the HMS Reaper in Cherbourg and shipped to the USA. The 240102 survied.
  HMS Reaper  The planes were cover with "Eronel". A Do 335 can be seen behind the two Me 262 and a Ta 152, the second behind the He 219. On the HMS Reaper the two Do 335, ten Me 262, two Ar 234, three He 219, nine FW 190, two Ta 152, three Bf 109, one Me 108, one Ju 338, one Ju 88, a Doblhoff jet-helicopter und two Flettner Fl 262 helicopters.
  Do 335 Werknummer 240102  the Do 335 Werknummer 240102 was stored outside in Washington. It's just a wreck...
  Do 335 Werknummer 240102   10.10.1974 the 102 was flown from New York to Frankfurt in a Boeing 747F.
  240102  The Do 335 Werknummer 240102 after the restoration.
The pictures were take in 1978, when the plane was displayed in München.

Thank you very much, T. Genth, for the pictures. His page is highly recommeded.
  DB 603E  The 12 cylinderengine DB 603E had 44,5 l and ca. 2000 PS.
  ejection seat  The ejection seat.
  Internationalen Luftfahrtmuseum Villingen-Schwenningen  A replika of the Do 335 is currently build up at the Internationales Luftfahrtmuseum Villingen-Schwenningen.