By Agence France-Presse
The as yet untitled project will be based on "The 15:17 to Paris: The True Story Of A Terrorist, A Train, And Three American Heroes" by friends Anthony Sadler, Alek Skarlatos and Spencer Stone, with journalist Jeffrey E. Stern, entertainment website Deadline Hollywood reported.
In August 2015, the trio -- one of whom was in the Oregon National Guard and another in the US Air Force -- foiled a gunman who had boarded a train from Amsterdam to Paris with enough ammo to kill 500 people.
They charged and overpowered 25-year-old Moroccan national Ayoub El Khazzani after he came out of a toilet armed with a Kalashnikov assault rifle, an automatic pistol and a box-cutter, shooting and seriously wounding a passenger.
The three have since been hailed as heroes both in the United States and in France, where they were awarded the Legion of Honor, the country's highest decoration.
El Khazzani is in custody in France, awaiting trial on various terror-related charges, including attempted murder.
The movie follows a pattern in Eastwood's recent output of recounting real life stories of heroism, following "American Sniper" (2014), about Navy SEAL sniper Chris Kyle, and "Sully" (2016), about "Miracle on the Hudson" airline pilot Chesley Sullenberger.
Industry daily Variety said Eastwood was hoping to get filming underway later this year.