The ZSO 523 was a proposed 1940s large military transport designed by the Zeppelin company of Friedichshafen. During the second world war Germany concentrated on building fighters and bombers and the production of support aircraft was transferred to occupied France. The project was abandoned when France was liberated in 1944.
With Messerschmitt concentrating on the design and build of combat aircraft the design of a larger transport/airliner based on the design of the large Messerschmitt Me 323 military transport was sub-contracted to Zeppelin. The aircraft was to be built in France by SNCASO.

The ZSO 523 would be like the Me 323 a high-wing monoplane but was larger with both a front and rear loading ramp. Powered by six 2200 hp Gnome-Rhône 18R engines it also had a six-wheel retractable main landing gear.

There is a bit more information on the ZSO.523 in the book Nieuport 1909-1950 (Docavia no.38) by Rosenthal, Marchand et al.
The study was done by a team led by Pillon and Robin. This was a team of former Nieuport engineers, integrated into the SNCASO in Cannes in 1941. The team was moved from Cannes to Paris and Issy-les-Moulineaux in late 1943, then they started to work on the Zeppelin project under threat of being sent to Germany.
A wooden mockup was built in the Levasseur factory (the book has a picture of the mockup). The prototype should have been produced by Zeppelin in Friedrichshafen, and the SNCASO would have participated in the series production.
It is said that the study was made at the lowest possible pace, while the team was secretly studying its own military projects. After the war, the Issy-les-Moulineaux team was transferred to the SNCAC, and one of these wartime secret studies became the SNCAC NC.1070 two-engine naval torpedo bomber. Experience gathered with the ZSO.523 project was used for the NC.211 Cormoran four-engine cargo, which indeed shows some similarity to the Gigant design.

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