September 11, 2001 was supposed to be a typical day for Lieutenant Heather Penney of the District of Columbia Air National Guard. As Penney recalled in a 2016 interview with HISTORY, that morning she was attending a briefing at Andrews Air Force Base, planning the month’s training operations. At about 8:45 a.m., someone leaned into the room and said, “Hey, somebody just flew into the World Trade Center.”

First Lieutenant Heather “Lucky” Penney had graduated from Purdue University, majoring in literature. She’d planned on being a teacher. When Congress opened up combat aviation to women, Penney immediately signed up. She wanted to be a fighter pilot like her dad, John Penney, a retired Air Force colonel who had flown combat missions in Vietnam and was now a commercial pilot for United Airlines. After her training, she was assigned to the 121st fighter squadron of the Air National Guard.

The morning of September 11th

Lieutenant Heather Penney. U.S. Naval Academy

The weather in New York City that day was very clear with blue skies. "We thought it was a small general aviation airplane or, you know, some small aircraft that maybe had...messed up their instrument approach," Penney recalled. It was assumed that a general aviation plane had made a terrible mistake, and they went back to their meeting.

Within a few minutes, there was another knock on the door, and someone said, “Hey, a second plane just hit the World Trade Center.” It was clear: America was under attack. They rushed to a nearby television and saw the burning towers. As Penney said, that was when "we realized that our world had suddenly changed."

READ MORE: September 11: Photos of the Worst Terrorist Attack on U.S. Soil

There was no time to arm their F-16 fighter jets

An illustration painted by Gil Cohen of an F-16 fighter jet, the same aircraft flown by Heather Penney, on September 11, 2001, flying over the burning Pentagon in Washington, D.C.. VCG Wilson/Getty Images

As confusion enveloped the briefing room, Penney's commanding officer, Colonel Marc "Sass" Sasseville, locked his eyes to hers and said, “Lucky, you’re coming with me.” They scrambled to the pre-flight area and donned their flight suits. There was no time to arm their F-16 fighter jets, so they would be flying this mission virtually unarmed, packing only their undaunted courage.

But what was the mission? Where were they to go? What were they looking for? There were no clear orders as to what to do. Somewhere in the confusion as the pilots got into their flight suits and ran to their planes, the Pentagon was hit by hijacked American Airlines Flight 77. Reports circulated that a fourth plane, United Flight 93 out of Newark, New Jersey, was out there. Air command speculated it was also headed to D.C. for another strike on the Pentagon, or a strike on the White House or the Capitol building.

Normally, preflight preparation for F-16 fighter jets takes a half-hour, allowing pilots to methodically work through a checklist. Being a rookie, Penney’s only combat experience was in training. As they ran out to their planes, she started going through the checklist. Sasseville stopped her and barked, “Lucky, what are you doing? Get your butt up there and let’s go!” She quickly climbed into her cockpit. As she powered up the engines, she shouted to the ground crew to pull the chock blocks holding the wheels.

Receiving the go-ahead from flight control, both jets’ afterburners belched out thousands of pounds of thrust as they took off and headed northwest, the last known location of the fourth plane. Word came to them that they had shoot-to-kill orders. Knowing that they had taken off with unarmed aircraft, that could mean only one thing. They would be flying a kamikaze mission, ramming into Flight 93, a Boeing 757 aircraft, nearly 7 times the weight of their F-16 fighter jets. They had agreed upon the plan of attack. Sasseville would head for the 757’s cockpit and Penney would aim for the plane's tail. As they sped out beyond Andrews Air Force Base, flying low at about 3,000 feet, they could see black, billowing smoke streaming from the Pentagon.

READ MORE: Behind the 9/11 White House Order to Shoot Down U.S. Airliners: ‘It Had to be Done’

Heather Penney’s dad might have been the pilot of United Flight 93

A display at the visitor center at the Flight 93 National Memorial on September 10, 2015 in Shanksville, Pennsylvania. Jeff Swensen/Getty Images

Beyond the mission at hand, there wasn’t much else on First Lieutenant Heather Penney’s mind. She had accepted the fate of Flight 93’s passengers, believing whether she succeeded or not, they were going to die. She briefly toyed with the idea of ejecting from her plane just before impact, but quickly dismissed the idea, knowing she had only one shot and didn’t want to miss. It didn’t even cross her mind that there was a possibility the pilot of United Flight 93 was her father, who often flew out of East Coast cities. As it turned out, he wasn’t.

For the next 90 minutes, Penney and Sasseville made ever-increasing sweeps of D.C. airspace, looking for the fourth airliner. "We never found anything," Penney told HISTORY. After about an hour into their mission, Penney and Sasseville heard that the Flight 93 had crashed in a field in Pennsylvania. Passengers on the flight had heroically prevented the hijackers from reaching their target.

Now the mission changed from intercept to sanitizing the airspace. Not every aircraft aloft that morning was aware the FAA had ordered a national ban on takeoffs of all civilian aircraft regardless of destination. With the assistance of civilian air traffic controllers, Penney and Sasseville began to divert any aircraft away from the D.C. area and ordered them to land as soon as they could. They also identified first-responding aircraft assisting the rescue at the Pentagon.

WATCH: The full special 9/11: The Final Minutes of Flight 93 online.

Penney and other pilots were instructed to guard the President of the United States as he flew home

At the time of the attacks, President George W. Bush was attending an elementary school event in Sarasota, Florida. When he was told a second plane had hit the World Trade Center and the country was under attack, he was escorted back to Air Force One and taken to the safest place at that moment, the open skies.

Now, in the evening hours, it was time to bring the president home. Penney's plane and the others patrolling the skies around Washington, D.C. had been equipped with live ammunition. They were also given “free-fire” authority, meaning pilots could make the decision to fire on any civilian aircraft deemed to be a threat, instead of waiting for authorization. Several hours after the initial attack, it was still unclear whether more attacks were pending.

WATCH: 9/11: Inside Air Force One on HISTORY Vault

Heather Penney was promoted to Major and served two tours in Iraq

Since that day, Heather Penney served two tours in Iraq, was promoted to Major, retired and currently works for Lockheed Martin Aeronautics Company. She has had time to reflect on her experience on September 11, 2001—and the bravery of the passengers on Flight 93.

"I made a decision with my life and I swore an oath to protect and defend, but these were just average, everyday people, mothers, fathers, school teachers, businessmen," Penney told HISTORY. "They're true heroes."