Pentagon spokesman George Little said the department's weeklong silence was a result of restrictions on the discussion of classified surveillance missions.
He said the administration wanted to handle the incident privately, and issued a stern complaint to Iran through diplomatic channels.
Washington and other world powers are preparing to hold a fourth round of negotiations with Tehran seeking to curb its disputed nuclear program. Those talks could take place as early as this month, U.S. and European officials say.
Republicans and Democrats in Congress were briefed on the Nov. 1 incident Nov. 2, a senior administration official told The Wall Street Journal. Congressional aides for both parties confirmed some staff members were briefed.
Republican presidential nominee Mitt Romney wasn't briefed, an aide told the newspaper.
"I think there was a reason that it didn't come out until November 8th," lawyer-diplomat Rich Williamson, who served as a Romney foreign affairs adviser during the campaign, told the Journal.
"I think that's because the people on the other side thought it would have political ramifications. It would raise issues they didn't want to address," he said.
White House officials declined any immediate comment.
The New York Times said disclosing the attack before Election Day would likely have been interpreted either as sign of the administration's weakness or as an opportunity for President Barack Obama to demonstrate leadership.
The U.S. Air Force MQ-1 Predator drone was flying in international airspace, 16 nautical miles from Iran, when it was shot at by two Russian-made Sukhoi Su-25 single-seat, twin-engine jets, known by NATO as Frogfoots, Little said.
The Iranian warplanes, under the command of Iran's aggressive Islamic Revolutionary Guard Corps Aerospace Force, fired at least two volleys of machine-gun fire at the remotely guided drone and missed the slow-flying drone, Little said.
After eluding the fire, the drone flew farther out to sea and turned to head to its base, Little said. The Iranian jets followed the drone for a period before breaking off, he said.
Little said the gunfire did not appear to be warning shots.
"Our working assumption is they fired to take it down," he said, adding: "Our aircraft was never in Iranian airspace. It was always flying in international airspace."
The shooting was the first known instance of Iranian warplanes firing on a U.S. surveillance drone, The New York Times said.
The U.S. protest was delivered to Iran through the Swiss Embassy in Tehran, which acts on behalf of U.S. interests in Iran, Little said.
The Defense Department plans to continue surveillance missions "consistent with longstanding practice and our commitment to the security of the region," Little said.
"We have a wide range of options, from diplomatic to military, to protect our military assets and our forces in the region and will do so when necessary," he said.
Tehran had no immediate public comment on the incident.
Defense officials said they were not sure why the warplanes crossed into international airspace and fired on the drone.
A senior administration official told the Times the episode should not be viewed as leading to a broader military confrontation with Iran. The official added the administration expected it would not derail the potential diplomatic contacts between the two countries over the nuclear program.
"We view the incident as problematic, but we're wary of the possibility of unintended escalation," a senior administration official told the Times.