the ultimate boondoggle of WW2 was the long-range General Motors/Fisher XP-75 Eagle fighter

the P-75 could have been classified as "a group of parts flying in formation."

In April of 1942, the famous designer Donovan Berlin of the Curtiss P-36 and P-40 was assigned the task of meeting the requirements for the new fighter. To keep down costs and shorten development time, he decided to use existing aircraft sub-assemblies already in production.

It was made up from sub-assemblies of other airplanes that were already in production. It used the tail of the Douglas SBD Dauntless, the wings of Curtiss P-40 Warhawk and the landing gear of the Vought F4U Corsair.

This methodology was chosen to reduce production times and costs, but unfortunately this "Rube Goldberg" approach of assembling parts from several different aircraft and getting a car parts maker to build it, didn't work out so well. What may have seemed like a good idea at the time, with good intentions to save time and money, only gave more credence to Murphy’s Law, “Whatever can go wrong, will go wrong.” The main problem was that each of these parts was designed to function as an integral unit to a specific airframe. Combing parts of different aircraft proved to be ludicrous.3

While it may be fun to poke at GM for the failure of the P-75, they have to be commended for the monumental effort in converting their eastern automotive division to aircraft production. This involved a complete tear down of its five eastern automotive assembly lines and its parts division, and completely rebuilding these facilities practically overnight.

The program was cancelled after only a small number of prototypes and production aircraft had been completed, as it was no longer required in its original role, could not be quickly deployed, and possessed no significant advantages over aircraft already in production.

or read a far better written, and thorough description here: https://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=420

https://en.wikipedia.org/wiki/Fisher_P-75_Eagle
http://www.aviation-history.com/fisher/p75.html
http://www.joebaugher.com/usaf_fighters/p75.html
http://mz-across-usa.blogspot.com/2018/05/the-national-museum-of-us-air-force.html