Although not very known, twins have been flying since 1915. The first was a design, named M9 or K1 (Kampfflugzeug), by the Dutchman Anthony Fokker. He lived at that time in Germany.

Fokker used two M7 fuselages and mounted them together to form the M9. The M7 was a biplane. On the outsides he placed the original wings and between both fuselages he placed a new wingsection as upperwing. The lowerwing was a combination of a wingsection and a short fuselage. This new fuselage had the original engines of both M7's. One was mounted in the front and one in the back driving a pusherprop.
Fokker placed in each M7 fuselage a gunner in the front (where now no longer the engine was mounted). Those gunners could now shoot next to the propeller. At those times you couldn't shoot through the propeller. Later Fokker designed a system that could.
The first flight was done without the gunners. The plane was very tailheavy and hard to control. This means that the CG (center of gravity) wasn't placed on its designed place, but more backwards. This isn't hard to believe when you know that the gunners were placed before the designed CG. So... flying without them shifted the CG backwards. Another problem during the first flight was that the fuselages rotated in relation to each other. The structure wasn't stiff enough to keep the fuselages nicely lined up.
Another flight was made with mechanics sitting in the gunnersplace. Flight behavior still wasn't good and the whole project was cancelled and the testplane scrapped.

Twins aren't always a failure.

Probably the only well known German twin: the Heinkel He 111 Z (Zwilling)

In World War II the Germans made a huge glider, the Messerschmidt Me 321 (see Giants). It was so large and heavy that 3 twin-engine Bf 110 fighters had to be used together to tow the glider in the air. These triple tows or "Troika-Schlepps" were very dangerous to all planes involved.
Generaloberst Ernst Udet, who did support several weird designs during WW II, came with the idea of making a twin, the He 111 Z (Zwilling), out of two Heinkel He 111's. This way he hoped to create a heavy transport that had the necessary power to tow the Me 321. Early in 1941 work began on two prototypes. They used He 111 h_6 fuselages and created a new wing section between the two inner engines. In this section they placed a fifth engines. Controls were divided between the fuselages. I quote one of my resources ("The Warplanes of the Third Reich" by William Green; Publisher: Doubleday and Co.): "Two completed undercarriages retained and the pilot was situated in the portside fuselage with five throttles, full instrumentation, controls for the port undercarriage members and the radiator flaps for the portside pair of engines. The second pilot, situated in the starboard fuselage, was provided with dual flying controls but no throttles and operated the starboard undercarriage members and the radiator flaps of the center engine and starboard pair of engines. A mechanic, radio-operator ang gunner were accommodated in the port fuselage and a mechanic and gunner in the starboard fuselage, the second pilot acting as navigator." I surely hope that the pilot with the throttles never got shot. Flying this twin from the other fuselage without using the throttles must be hard... let's even say impossible to land safe (personal finding).
Sometimes the five engines were not enough to tow the heavy loaded Me 321 and they needed to use a 502 kg (1 100 lb) thrust rocket beneath each fuselage and two 1507 kg (3 307 lb) thrust rocket beneath the centerwing, one on each side of the central engine.
Towing the Me 321 was done with a cable which was attached to a cable which was fixed to both inner wing-fuselage intersections. But each fuselage could have its own cable when smaller gliders were towed. There were succesful trials where 3 Gotha Go 242 were towed by a single He 111 Z.

The He 111 Z was operational since the summer of 1942, but all the planned operations were cancelled due to not obtained strategic goals like necessary captured airstrips. Their first operation was to deliver Me 321's filled with supplies to the troops in Stalingrad. Bad weather and in-between landings on overcrowded airfields delayed the flights and when they reached Makeyevka it was too late.
I would have liked to see the faces of the pilots, who were standing on these overcrowded airfields, when they saw these huuuuuge gliders coming in for landing. I bet most made a quick prayer to protect their parked airplane.
The first real operational use of the He 111 Z - Me 321 combination was getting casualties out of the Kuban bridgehead. He 111 Z's took off with 30 casualties aboard while towing a Me 321, which carried more than 100 casualties.
Many other operations were planned, but all were cancelled...again. This was not due to the airplanes, but due to the bad situation the Germans were near the end of the war.
Of the 12 delivered H 111 Z 4 remained at the end of the war. The rest was shot down or damaged during Allied bombing.

Another German twin was the Messerschmidt Me 609. It was formed by two Me 309-fighters. The Me 309 was intended as a replacement for the Me 109, which had no longer a superior performance in relation to the Allied fighters like the Supermarine Spitfire. It also lacked the necessary range. But the Me 309 development did progress very slow.
Only nine Me 309's were planned. The tests with the first Me 309 showed several teething problems such as overheating engines, wobbling and snaking groundhandling, high control forces. At the time when the 3th Me 309 was made, the Me 262 was already in production. The Germans saw no future in the further development of the "obsolete" piston-engine fighter.
But somehow there was a proposal of making a Me 609 out of Me 309's.

Why they made this proposal, what advantage it would have had ... I don't know. Could this Me 609 be related with the F-82 Twin Mustang (a Allied proposal which did start at the end of WW II)? Can anyone give me more details?

I would like to thank a person with callname "NiteWolf" for giving me the data on the Me 309.

Messerschmidt Me 609.
If Fokker had used a similar tail connection, his M9 would have been stiffer.

On 03-06-2001 I got a remark from Erik Bakker: " I read your page on twin aircraft, but I missed a section on the Messerschmitt Bf 109Z version. I noticed your questions of where the origin was for building the Bf 309Z. Perhaps this info is the 'missing link'. * Somewhere during 1941 work began on a experimental heavy fighter/ bomber version of the Bf 109 F. Two airframes were connected to a parallel chord center wing section to form a Bf 109Z. It was equipped with two 1.350 hp DB 601E-1 engines and had a span of 43ft 61/3in (13.27 m). The sole prototype was damaged before first flight in an allied attack. Perhaps this project got a second chance with the development of the Bf 309. Further (and much more detailed) info on the Bf 109Z can be found at http://world.std.com/~Ted7/bf1092.pdf. Some info in these article point in opposite directions but I'll not be the judge to that.
* 'Combat Aircraft of world war Two' written by Elke C. Weal, John A Weal and Richard F. Barker. with Editorial Consultant J. M. Bruce and written in co-operation with the RAF Museum in Hendon."

P51+P51=F82. Maths by a mad teacher? No no, just a description of another twin. During WW II the Mustang or P51 was known for its longer range. It could escort bombers to Berlin. But these flights were very long and a fighterpilot couldn't count on a co-pilot to prevent fatique. A tired pilot could make wrong and lethal decisions when encountering a enemy.
Now, what to do if the range of the bombers becomes even longer? This question became active when Consolidated Vultee B-24's (English version got callname "Liberator") and Boeing B-29 "Superfortresses" entered service in the Far East.

North American Aviation did propose a simple solution. A twin-version of the Mustang. Why not? It could be constructed in limited time because the P-52 tools could be used. Some changes had to be made. The original wings had to be reinforced, a new tailplane and a new undercarriage was made. The port cockpit had all flight and engine controls, while the other cockpit only had enough controls to land the plane in a emergency or to let the co-pilot fly while the pilot takes a rest (or nap).

The Twin Mustang or F-82 with a central radar pod in a much used total black outfit.
It sure must have been a scary experience for the enemy.

The reason for the very long pod can be found in the interference of the props. To keep the radar free of this interference it had to placed before the props. A very long pod was the solution.

If there is one airplane that could deserve the name Chameleon it would be the Piper Cub. The Cub has been changed in its history to many new needs. I have no idea what need did lead to the next variant of the Cub.

A twin-variant of the Piper Cub. Note the props which are placed VERY close.

Can somebody tell me why they did make this twin. Were they in need of a larger cargospace? Did they want a 4-seater? Did they want 2 pilots in the front? You tell me.

Another twin I found was the Fouga C.M. 88-R "Gemeaux" (Twin). It is a twin version of the Fouga C.M. 8 Cyclope. It uses its fuselages, its typical V-shaped tails and the outer wings. They made a new wing section between both fuselages and made a connection under the tails (it is just visible on the frontal picture). This twin was a testbed for the light Turboméca jet engines.

The Fouga C.M. 88R "Gemeaux".

A frontal view on the "Gemeaux".
Just visible is a connection under the two V-tails.

My opinion about twin:

hmmm....seeing these examples I think that twins were only used as hasty solutions for a typical temporary problem. Assembling a twin was done much faster than designing a completely new airplane and making the necessary tools and rigs.

But they also had some disadvantages.

The F-82 had two single cockpits. The co-pilot couldn't help the pilot if he got shot. He could only fly the F-82 to the nearest base. Once there the first aid could be done... maybe too late.

The He 111Z had twice the internal volume of the H111. But I bet that a new freighter with the same total volume could carry larger parts. Just look at the Super Guppy (see "Giants"). A wider fuselage gives the possibility to load larger parts.

Imagine if they would even have used the He 111Z as a bomber. It would be a BIG problem if one fuselage had a release problem during the bombing. The asymetrical load would flip the airplane on its back.

One thing still puzzles me. Fauvel, a French airplane manufacturer specialised in unswepted flying wings (go see the Fauvel section in the Nurflugel site), did propose some military airplanes. I found some pictures of them in the book "Les Ailes Volantes" by Alain Pelletier (ISBN 2-7268-8444-X). It mostly are twins. But he didn't use existing fuselages. Every project had a new fuselage (actually two of the same design). Why????? Did he attempt to reduce the side area to prevent being hit by fighters? I have no clue. Can anyone help me clear this one out?

Another similar project was the Arado P530 (go see www.luft46.com) . It enhances two newly designed fuselages. But this project got cancelled because it had no advantages when compared to the H 111 Z. It even had disadvantages, due to the fact that the H 111 Z had very few new parts, most were stock H 111 parts. I guess that this would have killed all the Fauvel asymmetrical proposals before they left the drawingboard.

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