Emma Scanlan, an attorney for Robert Bales, made the request during a hearing Tuesday at Joint Base Lewis-McChord south of Seattle.
Citing attorney-client privilege, Scanlan did not say why the request was made. The defense team provided its reasons to the judge - but not prosecutors - in a confidential court filing.
Prosecutors objected to the motion, saying it smacked of witness shopping.
Outside experts believe a key issue going forward will be to determine if Bales suffered from post-traumatic stress disorder. Bales served tours of duty in Iraq and Afghanistan.
A ruling on the defense team's request will be made later.
At Tuesday's hearing, attorneys also discussed which witnesses might be allowed to testify on Bales' behalf, should the case reach a sentencing phase.
"A lot of our witnesses that we want to testify in a potential penalty phase will be here, including Sgt. Bales' mother," Scanlan said. "She's a very important witness to who he was as a child and who he is a man."
Defense attorneys also asked for a consultant to be appointed to help them pick jurors. The judge said he would rule on that later.
The defense also requested the handwritten notes of the first Afghan government officials who viewed the crime scene.
The defense team has received an official report about those findings, but lawyers said the notes could yield information left out of the report. Prosecutors said they so far have been unable to obtain the notes from the Afghans. At the judge's request, they agreed to make another attempt through official channels.
"They took a lot of notes, and that's what we want to see," Major Greg Malson, one of Bales' attorney, said after the hearing.
Bales is to be court-martialed on premeditated murder and other charges in the attack on two villages in southern Afghanistan.
The Ohio native and father of two is accused of slaying mostly women and children during pre-dawn raids on March 11, 2012.
Bales, 39, has not entered a plea. The Army is seeking the death penalty. The U.S. military has not executed anyone since 1961.
The slayings last year drew such angry protests that the U.S. temporarily halted combat operations in Afghanistan, and it was three weeks before American investigators could reach the crime scenes.
Bales' defense team has said the government's case is incomplete.
During a previous preliminary hearing, prosecutors built a strong eyewitness case against the veteran soldier, with troops recounting how they saw Bales return to the base alone, covered in blood. One soldier testified that Bales woke him up in the middle of the night, saying he had just shot people at one village and that he was heading out again to attack another. The soldier said he didn't believe Bales and went back to sleep.
Afghan witnesses questioned via a video link from a forward operating base near Kandahar City described the horror of that night. A teenage boy recalled how the gunman kept firing as youths scrambled, yelling: "We are children! We are children!"
An Army criminal investigations command special agent testified earlier that Bales tested positive for steroids three days after the killings, and other soldiers testified that Bales had been drinking the evening of the massacre.