Hildebrand will drive the No. 21 Chevrolet for ECR. The team said Thursday a sponsor will be announced at a later date.
Team owner Ed Carpenter will drive ECR's other entry in his IndyCar season debut. Carpenter is only racing on ovals in 2014. He won the pole for the Indy 500 last year.
Hildebrand nearly won the Indy 500 as a rookie in 2011 but crashed exiting the final turn. He was passed by Dan Wheldon and finished second.
He drove for Panther Racing for three years, but was released after last year's Indy 500.
The ECR two-car effort is the only all-American entry currently entered in this year's Indy 500.