Jaguar wasn't used to losing, having won five times in seven years at the 24 hours of Le Mans through 1957. When the rules changed for the next season, effectively obsoleting the D-Type, it ended Jaguar's attempts in sports car racing. In the 1960s, they did develop the XJ13, even taking it so far as to test it, but the project was quietly dropped.
With the Coventry-based company not interested, that meant if the Jaguar marque was going to go racing, it was up to privateers to make it happen. Enter Bob Tullius. His team, Group 44, first started racing Jaguars in 1974. He won the SCCA B Production category with a lightly modified Jaguar E-Type V12.
Over time, he kept utilizing the V12 Jaguar in increasingly larger series, including the Trans-Am series. So when IMSA announced a new GTP class, Tullius made a decision to build a prototype racer around the V12 he so expertly understood.