AMNA NAWAZ: All week, we're marking the anniversary of 9/11.

Twenty-five years ago, the passengers of United Airlines Flight 93 stormed the cockpit, forcing the hijacked jet to crash in Shanksville, Pennsylvania.

But before the world knew of their bravery, members of Washington, D.C.

's Air National Guard were prepared to make their own sacrifice.

Nick Schifrin has one of their stories.

HEATHER PENNEY, Former Pilot, D.C.

Air National Guard: On that day, ordinary people rose to extraordinary things, and we saw the best of who we can be as America.

NICK SCHIFRIN: Twenty-five years ago, Heather Penney was a pilot in the Washington, D.C., Air National Guard, call sign Lucky, and she felt lucky to be the first female Washington, D.C., Air National Guard pilot and continue the family legacy.

HEATHER PENNEY: My daddy was a fighter pilot.

He flew A-7s in Vietnam.

He did bombing missions over North Vietnam.

And as a little girl, I actually found some of his mission tapes from Vietnam.

It was the most harrowing, exciting, sort of meaningful thing that I could think of.

And so I was just on fire to be a fighter pilot.

It wasn't until I was in graduate school that Congress had opened up combat aviation to women.

First thing I did was apply to the Air National Guard.

WOMAN: It's Tuesday, September 11, 2001.

NICK SCHIFRIN: And 25 years ago, she was on duty as the day began under a blue sky.

HEATHER PENNEY: Oh, gosh, the sky was so blue.

It was crystal blue, and it was a perfect flying day.

Tech Sergeant Dave Callahan knocks on the door and he says: "An aircraft just hit the World Trade Center."

We assumed it had to be a Cessna, like a little airplane, bounced off the building, right?

We didn't take it seriously.

And it wasn't until he came back a second time, he said: "A second aircraft hit the other World Trade Center, and it was on purpose."

And that's when we knew our nation was under attack.

WOMAN: Lord Jesus!

NICK SCHIFRIN: By then, the towers had fallen.

The Pentagon was hit.

And they thought more hijacked planes were flying toward Washington, D.C.

PETER JENNINGS, ABC News: There are indeed reports that there are possibly one or two more planes that have been hijacked.

NICK SCHIFRIN: Penney and her wingman, then-Lieutenant Colonel Marc Sasseville, were ordered in the air.

In an oral history, your commander, Brigadier General David Wherley, remembers that he spoke to you and Colonel Sass, he's known, and said "You have weapons-free flight lead control.

Do you understand what I'm asking you to do?"

What did he mean?

HEATHER PENNEY: We had the authorization to use lethal force.

The problem was Sass and I didn't have weapons.

Missiles had not gotten to the flight line yet.

We would have had to ram our jets into what we then knew was the target, an airliner.

NICK SCHIFRIN: A civilian airliner.

HEATHER PENNEY: A civilian airliner with innocent Americans on board.

NICK SCHIFRIN: Did you pause?

HEATHER PENNEY: No.

We had seen how the terrorists had aimed for the towers.

We could see from the flight line the smoke from the Pentagon.

We knew that if any other terrorist was headed towards D.C., the heart of our nation, the capital of the free world was in danger.

And we needed to do whatever we could to protect that.

We also knew that the fate of those innocents was already sealed.

There was nothing we could do about that.

NICK SCHIFRIN: Did you and Sass have a plan?

HEATHER PENNEY: So what Sass would do is, he would ram his jet into the cockpit, take out the terrorists, take out the flight controls, ensure that the engine controls the throttles, that that was absolutely gone.

And then I would -- I would hit the tail.

I would aerodynamically destabilize the aircraft and it would go straight into the ground.

NICK SCHIFRIN: The way you just described that is the technical means that you and Sass were thinking in the moment.

But it was also a suicide mission.

HEATHER PENNEY: It would have been a suicide mission.

And neither one of us hesitated.

Everyone on the base, any individual who had ever raised their right-hand would have wanted to be in that two-ship to protect our nation.

It was my job.

And if God had put me in that place at that moment, then that was my purpose.

And that has never changed.

But I will say this.

Sass and I were a mission failure because we didn't get launched in time, because we didn't reach 93 in time, because the Pentagon was hit.

There were so many failures in the system because our nation wasn't ready.

NICK SCHIFRIN: Today, she tries to ensure the nation is ready, as the director of studies and research at the Mitchell Institute, a think tank focused on airpower.

HEATHER PENNEY: In the mid-90s, when the nation cut our Air Force in half, which was the reason why I was sent on a suicide mission, we have gotten even smaller.

The F-16 that I scrambled with on that day is still in the air force inventory.

It is still flying.

The young men and women, those airmen that are going and doing what our nation has asked them to do are doing so in aircraft that are older than they are.

I do what I do today at the Mitchell Institute so that I can advocate for what our airmen need so that they are not also sent on a suicide mission.

JOHN PENNEY, Father of Heather Penney: It's an honor to be here this evening as a father of a fighter pilot.

NICK SCHIFRIN: And Heather and her father John have even spoken together about their careers in aviation and how their paths could have crossed on 9/11.

What did your daddy do after he was in the military?

HEATHER PENNEY: My dad, after he left the Air Force, became a pilot for United Airlines, and he was a captain for United Airlines on 9/11.

NICK SCHIFRIN: Did it ever cross your mind that he could be piloting 93?

HEATHER PENNEY: It was the equipment he was flying.

He had been flying up and down the East Coast through August and through September.

But it never crossed my mind that he could have been one of the pilots.

But that would not have mattered.

Those pilots were already dead.

I had a job to do.

NICK SCHIFRIN: A job, an ultimate sacrifice that she didn't have to make, thanks to the courage of the passengers of United 93.

The iconic line connected to United 93 is "Let's roll."

NICK SCHIFRIN: The last known word spoken by Todd Beamer before he and the other passengers stormed the cockpit.

What do you think about that today?

And how do you relate to that courage and sacrifice?

HEATHER PENNEY: I still get chills down my spine.

What they were willing to do that day gives me hope.

Not only am I intensely grateful for their sacrifice and what they were willing to do for all of us.

Not a single passenger on 93 was a service member.

There were no firefighters.

There were no EMT nurses.

There was no law enforcement.

They were just ordinary, everyday Americans.

And yet they were willing to fight the terrorists, to seize control of the cockpit and sacrifice their lives to protect us.

It proves to me that heroism isn't something unique, that that exists inside every single one of us.

NICK SCHIFRIN: Heather Penney, it's an honor.

Thank you so much.

HEATHER PENNEY: It is a privilege to be here.

It has been a privilege to serve my nation.

And thank you for helping remember what happened on that day.