Arado Ar.240 - Prototypes and early versions.

The Arado Ar.240 was a German twin-engined heavy fighter developed during the Second World War by the Arado Flugzeugwerke for the Luftwaffe.

Initially it was the Arado's response for the 1938 second generation Zerstörer contest in order to get a fighter that could replace the Messerschmitt Bf.110 which was already becoming outdated. Arado and Messerschmitt placed their candidates, being the Messerschmitt candidate the Me. 210 which was a totally new design that thanks to Messerschmitt experience developing the Bf.110 it was able to enter into service quickly. Being Arado a smaller company than Messerschmitt, their design was really ambitious and was the dreamed project of Walter Blume, the Arado's chief designer. Even if the deliveries would take some time, the RLM was interested enough in this design to order some prototypes.

Before that contest, Arado had invested in several lines of basic research, one of their developed lines was the "Arado travelling flap" which delivered excellent low-speed lift performance. Another development line was the investment into pressurized cockpits which decreased pilot's fatigue above 4500m. Finally, they also invested heavily in remote controlled turrets that, thanks to the gunsights placed both on top and bottom of the aircraft, it allowed aiming in any direction. They decided to combine all those inventions into one single airframe.
In order to achieve outright performance, they employed the smallest wing reasonably possible lowering that way the parasite drag, even if it meant the increase of the lift induced drag. Normally that would've meant that the airplane would have impossibly high landing speeds but it was offset by employing a huge travelling flap and leading edge slats for high low-speed lifts.

It was powered by two 1175hp inverted V-12 liquid cooled Daimler-Benz DB 601 inline engine with a three-bladed propeller. The radiators were a copy of the Junkers Ju.88 and they consisted in an annular block placed in the front part of the engine. That made the engine to look like a radial engine instead of an inline one. This system was later used in the Focke-Wulf Fw.190D, Ta.152 and in the twin-engined Ta.154.

Fuel cells had a self-sealing system that employed thinner tank lines allowing that way more fuel capacity and the airplane was also required to have dive bombing capabilities. Given the thick wing panneling, the aircraft wasn't suitable for conventional air brakes systems, so a "petal"-style was added at the tail of it, just like the one that had already been tested in the Do.217.
The cockpit was fully pressurized, thanks to the remote-controlled turrets, because that way the cockpit wasn't "penetrated".
Finally the airplane had to support a very high wing loading of 330 kg/ per square meter, compared to the average of 100 in a common fighter.

In May 1939 the RLM ordered six prototypes:

• Ar.240V1: The very first prototype. First flown on 25th June 1940 and proved to have poor handling in all axes and tended to overheat when taxiing. 
• Ar.240V2: Not drawn due to the lack of sources. However it's known that had larger thick wings and an additional vertical fin area in the dive brakes to reduce the yaw. It was also equipped with additional radiators to the gear legs in order to improve the cooling. It flew for the first time on 6th April 1941 and was used for experiments in the Arado factory.
• Ar.240V3: The first one to be equipped with the FA 9 rear-firing armament system. Such system was developed jointly by Arado and DVL and was armed with a 7,92mm MG 81Z machine gun.
• Ar.240V4: AKA Ar.240 A-01First one to include an operational dive brake.
• Ar.240V5 & V6:AKA Ar.240 A-02 Flown in December and January of 1941 and 1942 respectively and featured the FA 13 armament system which used two 13mm MG 131 machine guns instead of the MG 81Z in order to increase the firepower. 
• Ar-240 A-03: A reconnaissance/light bomber pre-production version.

The A-01 and A-02 were assigned to the 13. "Zerstörer"/Jg.5 based in Petsamo, northern Finland, where they were employed in reconnaissance missions over the railway lines leading to Murmansk.

The A-03 were assigned to the 3.(F)/100 and operated along the Ju.88 from February 1943 until July 1943 when passed to served under Luftwaffen-Kommando Don.

Some other are said to have served with the 1.(F)/100 which was fully incorporated in the Luftflotte 6 where it's claimed that, stripped from their armament, they flew reconnaissance missions over England in 1942, where no other German two-seater could venture in by 1942.
By the summer 1943 all those belonging to the Jg.5 were transfered to the 2.(F)/122 based in Frosinone, located South-East from Rome where this airplane saw it's first combat action but it was so badly damaged that was destroyed during landing. The continual problems with the machinery made the squadron staff to send the airplanes back to Germany without taking more fighting.

Sources:
1. https://en.wikipedia.org/wiki/Arado_Ar_240
2. Salamander Books - The Complete Book of Fighters
3. http://www.sas1946.com/main/index.php?topic=35260.0