Five soldiers with the 101st Airborne Division have been charged in the rape and killing of Abeer Qassim al-Janabi in Mahmudiyah, south of Baghdad. Her mother, father and six-year-old sister were also shot dead.
Howard told the court that he was left behind at a checkpoint while four other soldiers went to rape the girl, and that he had overheard the four planning the attack.
"I was slowly starting to believe what they had done, that they had committed the crimes, the rape and the murder," Howard said.
He reportedly said in a statement to the court: "If I could go back, I would not have let it happen in the first place. I definitely would have told somebody."
The Mahmudiya case was one of several killings and abuses involving US soldiers that prompted Iraqi officials to call for a review of foreign troops' immunity from prosecution in Iraq.
Two soldiers had previously pleaded guilty to the rape and murders in Mahmudiyah.
Last month Sergeant Paul Cortez was sentenced to 100 years in prison for his part in the rape and murders, while specialist James Barker was sentenced to 90 years in prison last November.
The alleged ringleader, Steve Green, was earlier discharged from the army and is to be tried in a federal court.