The P2V-7 then crashed into the mountains about 700 feet off the lead plane's flight path, the report noted.
NTSB spokesman Keith Holloway said Wednesday the agency is still investigating the cause of the June 3 crash, including potential mechanical failure, pilot error and a probe into whether the plane's deviation from the lead aircraft's flight path may have been a contributing factor.
Complete findings likely won't come for months. The preliminary report did not cite any causes.
Experts say a lead plane is typically used to guide the larger tanker into the drop zone above a blaze.
Tanker pilots are supposed to always keep the lead plane in sight, but that can be difficult in smoky conditions close the ground where wind can push the plane around, said Thomas Eversole, executive director of the American Helicopter Services & Aerial Firefighting Association.
"Generally, you won't find anyone who deviates from that flight path unless they lose sight" of the lead plane, Eversole said. "If he loses sight of him, he could very well get himself into a position where he could be in a lot of danger.
"The wind from those fires does really crazy things," Eversole added. "It could have pushed him sideways just enough to get him off course, and if he lost sight of his lead plane, that could have been it."
Firefighters were battling a lightning-sparked wildfire that jumped the Nevada border about 150 miles northeast of Las Vegas when the crash occurred. Investigators said the plane was reduced to fragments upon impact. Two pilots from Boise, Idaho, were killed.
The crash renewed concerns about the nation's aging firefighting aircraft fleet, along with the dwindling number of available planes just as the West enters what is expected to be a brutal season, and massive blazes burned in Colorado and New Mexico.
The U.S. Forest Service doesn't own any of the firefighting aircraft, but instead contracts with private companies that have limited stocks. This week, the service announced it was increasing the national fleet to 17 available aircraft. And on Wednesday, President Barack Obama signed a bill aimed at speeding the contracting of the next generation of air tankers, eventually moving away from the aging fleet now available.