The origins of the 51TH are a little fuzzy.

Some old CMDRs from it says that the corporation was created in 1991, before the Third World War in 2040.
Due to the loss of massive amount of informations after the datacenters and information storages' destruction, it is quite hard to confirm this...

The lone image we have is an old 2D photography of their main headquarters, based in what it was the city of Marseille, in France, in the very beginning of the 21st Century.

All we can say, is that the 51st Wing TH Massilia, abbreviated 51TH, TH for "Tous Humains" (all humans in old french), is a corporation of mercenaries who decided to work together as one entity.
We don't know how exactly how many they are. We just know they are all the descendants of top mil-spec fighting pilots and CMDRs from all horizons, able to fight in whatever has wings, and state of the art independant scientists and explorers, working in total independance for their own goals and objectives.

They are promoting justice and equality -under their rules-, by force if necessary...

Conceptors of the Arena:51, months before CQC.

Some old CMDRs, all around the bubble said that it is preferable to be in good shapes with them...