This is a topic that I've been thinking about for a while. I'm only including airplanes from the 1940's that flew, but deleting mid-engined airplanes such as the Bell P-39 and P-63.

6 airplanes, that I know of, meet my criteria; the Vultee XP-54 Swoose Goose, the Curtiss XP-55 Ascender, the Northrop XP-56 Black Bullet, the Dornier Do-335 Pfeil, the Kyushu J7W1 Shinden, and the SAAB J-21.

5 of the planes have pusher propellers; the Do-335 has both a pusher and a tractor propellers.

The XP-54 and the J-21 have the empennage supported by twin booms aft of the propeller; the XP-55 and the J7W1 have a canard configuration; the XP-56 has no empennage; and the Do-335 uses a cruciform empennage forward of the pusher propeller.

The 3 American designs were originally to be powered by the P&WA X-1800 engine, but that program was cancelled other engines were substituted. These designs were in response to a 1939 Air Corps requirement R-50C, which called for an advanced design of unconventional configuration. None of these programs showed much promise and they were all cancelled.

The J7W1 was designed as an interceptor and was planned for both reciprocating and jet power. The end of WW II closed out this program.

The Do-335 was the most promising design reaching speeds of over 470 mph. Some reports have this airplane as the fastest reciprocating airplane of WW II. Again WW II ended the program.

The J-21 is easily the most successful design of the group, being placed in squadron service with the RSAF as the J-21A. Later the airplane was converted to jet power as the J-21R and it also saw service with the RSAF.

Examples of the XP-55, the XP-56, the Do-335, and the J7W1 are part of the National Aeronautics and Space Museum division of the Smithsonian Institution.

A J-21A is on display at the Swedish Air Force Museum near Stockholm.

Links to webs sites on these airplanes are;

http://www.wpafb.af.mil/museum/research/p54.htm

http://www.nasm.si.edu/research/aero/aircraft/curtiss_xp55.htm

http://www.nasm.si.edu/research/aero/aircraft/northrop_xp56.htm

http://www.nasm.si.edu/research/aero/aircraft/dornier_do335.htm

http://www.nasm.si.edu/research/aero/aircraft/kyushu_j7w1.htm

http://www.ipmsstockholm.org/magazine/2000/02/stuff_eng_detail_j21.htm

There was the Ryan Fireball...had a piston engine (radial) up front along with a jet engine. Just missed being used in WWII. A couple of websites with some information...

http://www.warbirdsresourcegroup.org/URG/fireball.html

http://www.aviation-heritage.com/eZine/Col03.htm

Im thinking the F-82 Twin Mustang was pretty unconventional as well...

Greg

There was also a weird version of the F 84 that was the only turboprop aircraft to have an afterburner.

Here is a link to more info:
http://www.abc.net.au/science/k2/moments/s934718.htm

The flying pancake was one of the weirdest, but there were a LOT of weird ideas in the late 1940s.
I'm not at home right now or I'd open up my book about combat aircraft prototypes since 1945 which shows a lot of them.

Ahh yes, the XF-84H. Talk about beating a dead horse.

If you go into the 50's you have the highly unconventional XFY-1 Pogo Stick and XFV-1, the twin contra rotating turbo prop tail sitters that took off vertically then transitioned into horizontal flight.

Here in the UK the Gloster Meteor was fitted with props. I found this :-

http://tanks45.tripod.com/Jets45/Histories/Trent/Trent.htm

Hope it works  

Sorry, the Do 335 was NOT the fastest piston-engined fighter developed during World War II. That distinction went to the North American P-51H Mustang (which started to enter service near the end of the war) with a top speed of 487 mph and the Supermarine Spiteful, which almost became operational with the RAF with a top speed of 494 mph.

If I'm correct there was a XB42 that had contra pusher props. Anybody with info on this??

Yes Jetmek, Douglas made 4 XB-42's, called the MixMaster. Twin engines in the fuselage driving 2 contra rotating pusher props.

HaveBlue,
Didn't the XB-42 evolve into another airplane? I thought the Mixmaster was the Cessna O-2????

the XB-42 evolved into the all-jet XB-43.

The O-2 was called Birddog (maybe Mixmaster was an unofficial name for it though).

Jwenting is correct on the XB-42.

But, the O-1 was called the bird dog. It was a single engine Cessna tail dragger... a light observation plane used for spotting and calling in airstrikes.

The O-2 is a military version of the Cessna 337, which is a push pull twin engine Cessna with tricycle landing gear, and two booms for tails in between which is housed the pusher prop (think P-38 tail).

And though the O-2 and the XB-42 are both twin engine prop planes, that's where the similarity ends. The XB-42 had both engine in the fuselage, with the 2 props inline with each other and contra rotating... both at the very tail of the plane and pushing.

Bomber launched parasite fighter

ah yes, got confused between those 2  

Keesje, that's no bomberlaunched parasite fighter. It's a light bomber/observation aircraft.
http://www.luftarchiv.de/flugzeuge/blohm-voss/bv141.htm German text only.
Technically not 1940s either as it enteres service in 1937