In 1943 the USAAF was looking for a high-performance interceptor and someone suggested the bright idea of using parts from several different aircraft to build a new one and using an auto parts maker to do it. The Fisher division of General Motors was selected to combine parts of a P-40 with a Corsair and Dauntless to create the XP-75 Eagle which was a disaster! The thinking here was to save money with combined parts and using Fisher for assembly for mass production. 15 XP-75s were built and problems emerged immediately with the twinned Allyson V-3420-19 engine that was mid-fuselage mounted behind the pilot like the P-39 & P-63, driving an extension shaft to power contraprops up front. The shaft caused vibration problems and the engine tended to overheat. Climb time was slow and max speed was just 400 mph (good enough for 1943 but insufficient for '44-'45). Since the USAAF changed its requirement from interceptor to escort fighter to replace the P-51 eventually, the Fisher design was modified with a better tail, more powerful version of engine, a bubble canopy, more speed, elimination of vibration, and armed with 10x .50 cals (4 in the nose + 6 in the wings)! It still used the P-40s outer wing panels and the Corsair's gear, but was a vast improvement. So an order was placed for 2500. Only 15 XP-75s were made and just 5 of the improved P-75As before the order was cancelled. The P-51D emerged as the best USAAF escort fighter overall so there was no need for the P-75 Eagle projected for 1945 as war with Germany was anticpated to end by Xmas '44. The Germans fought on for 4 months and 7 days longer. The P-75 Eagle was known unofficially as the "Bitzer" due to bits of different aircraft. Had it come into battle in the ETO the LW would have been able to kill it with the Fw-190D-9 series and Ta 152 and the Bf 109Gs and K-4 too. Despite its armament (the most of any fighter other than the He-219 Uhu), it was slow at 412 mph, could not reach high altitudes fast enough, and had poor agility. It was more like a heavy weapons platform mid-air...

The POS original design:
http://upload.wikimedia.org/wikipedia/commons/5/5d/Fisher_XP-75_061024-F-1234P-040.jpg
The improved P-75A design:
http://www.militaryfactory.com/aircraft/imgs/fisher-xp75-eagle.jpg
Looks impressive, but was not!!!