Back in January, the 1968 Ford Mustang that starred in Bullitt made history when it became the most expensive Mustang ever sold at auction, selling for a record $3.74 million and beating the previous record held by a 1967 Shelby Mustang GT500 that sold for $2.2 million. It's going to take a very special Mustang to beat the record, so we thought the Bullitt Mustang would hold its title for a very long time. However, organizers at Mecum Auctions believes this very rare Mustang Shelby GT350 R prototype could set a new auction record when it goes under the hammer in May.

What makes this Shelby Mustang GT350R so special? Known as the "Flying Mustang," this is the very first Mustang Shelby GT350R ever built.

It's recognized as the most historically important Shelby Mustang in the world and arguably the most important Mustang ever made among Ford coupe lineup. Driven by Ken Miles, who was portrayed by Christian Bale in 'Ford v Ferrari' last year, 5R002 represents the first competition Shelby Mustang and the first Shelby Mustang to win a race. It also helped spawn the second team car (5R001) and all 34 customer R models.

The Shelby Mustang GT350R made its competition debut at Green Valley Raceway in 1965, where it achieved its first race win and was famously photographed in mid-air. This dramatic photo was used in marketing campaigns to promote Shelby and helped changed people's perception of the Mustang as the Shelby Mustang GT350R continued to dominate the race track.

It was so successful that Shelby used it as a factory demonstrator to show potential clients the capabilities of an R-Model. Aesthetically, 5R002 is also completely unique since it's the only car built to its specific configuration with numerous parts and design features not found on any of the R-models that succeeded it.

Shelby sold the first GT350R to Bill Clawson, a Ford Performance Division engineer, for $4,000, who kept extensive documentation of the car. After a couple of engine failures, Clawson enlisted the head of Ford Performance Division to install a full racing-spec GT40 engine, which also happened to be a Ford "XE" experimental engine that was never supposed to leave Ford to prevent GM from obtaining it. This same engine is still under the hood today.

The iconic Shelby then changed owners several times being meticulously restored to its original racing configuration as seen at Green Valley, Texas, in February 1965 by a Shelby collector in 2010, which took several years. The restored 5R002 debuted at the 2014 Amelia Island Concours to coincide with the Mustang's 50th anniversary, where it won a Best in Class award.

The prototype Shelby Mustang GT350R will be sold at the upcoming Mecum's 33rd Original Spring Classic auction taking place in Indianapolis between May 12-17. To become the most expensive Mustang ever sold at auction, it will need to fetch over $3.74 million.