Iran and Boeing go way back. Boeing was the largest supplier of civilian aircraft to Iran before the country's 1979 Islamic revolution. And despite the fraught relations between the U.S. and Iran since then, Iran has kept flying those planes for decades.
As part of the recent Iranian nuclear deal and the lifting of sanctions, Boeing is once again permitted to sell planes to the Islamic Republic. And Iran desperately wants to start replacing its fleet of aging, worn-out commercial aircraft.