This remarkable aircraft was designed in response to a 1938 specification from the Italian Air Ministry by Luigi Stipa and followed on from his ducted fan proof of concept the Stipa-Caproni. The only information I have been able to find on the SAI 404 is a page on the Italian Wikipedia which suggests that a prototype was actually built and flown (with difficulty) in 1940-41 and that the tandem 840hp V12 engines tended to overheat.
The SAI 404 was quite a large aircraft and is loaded weight would have been approximately the same as an empty B17 (16,000kg) but with a 9,000kg payload compared to the B17's 8,000kg payload. The model is correspondingly large and because of it's unconventional shape requires more material to 3D print than a more conventional design.
The Spherical depression underneath the model is for mounting it on a 6mm BB.