The Army's contracting notice says that only two complete guns systems would be available in total, suggesting that the Army could ask for them back after a certain amount of time in order to send them to other competitors.
As it stands now, likely entrants include General Dynamics Land Systems Griffin III, a version of the KF41 Lynx from Rheinmetall and Raytheon, and a variant of BAE Systems' CV90. The Army has also recently reviewed Germany's Puma armored fighting vehicle.
The Griffin III already packs the XM913 gun. At least at present, the Army is not officially mandating that proposed designs use this cannon, but has reportedly made it clear that they would prefer that the vehicle carry some sort of 50mm weapon.
General Dynamics' Griffin III with its 50mm XM913 gun at maximum elevation.Photo by: Joseph Trevithick
The XM913 is a fairly mature design. The gun is a refined example of the Bushmaster III automatic cannon design, which Alliant Techsystems, or ATK, first brought to market more than a decade ago. The Bushmaster III itself had leveraged decades of work done on earlier examples of the Bushmaster family, including the 25mm M242 cannon on the Bradley Fighting Vehicle. In 2014, ATK announced that its aerospace and defense components would merge with Orbital Sciences Corporation, which resulted in the creation of Orbital ATK. Northrop Grumman bought this company in 2018.
The initial Bushmaster III was as 35mm cannon, but was convertible into 50mm with minimal modification by using ammunition that fired larger diameter projectiles, but that used same cartridge base dimension. The Army, for a time, considered potentially up-gunning the Bradley to use the 35mm version.