Bierenbaum, an experienced pilot who had been convicted on circumstantial evidence, was serving his 20 years-to-life prison sentence when he made the chilling confession during a December 2020 parole board hearing.
It was the first time he had admitted to the crime since his wife, Gail Katz, disappeared in 1985. Her body has never been found.
"I wanted her to stop yelling at me and I attacked her," according to a transcript of the hearing obtained by ABC News.
When asked how, Bierenbaum said he "strangled" her, and went on to say, "I went flying. I opened the door and then took her body out of the airplane over the ocean," the transcript said. He also told the board he killed his wife because he was "immature" and "didn't understand how to deal with his anger," according to the transcript.
His admission shocked everyone involved in the case, because it was the exact theory prosecutors had presented in court at Bierenbaum's 2000 murder trial.
"I was like, 'Holy s---, are you kidding me?'" said Dan Bibb, one of the prosecutors who brought Bierenbaum to justice. "I was stunned because I always thought that that day would never come, that he would own up, take responsibility for having killed his wife."
Bierenbaum remains in prison. His next parole hearing is scheduled for November.
Watch the full story on "20/20," now streaming on Hulu.
When Gail Katz met Bierenbaum in the early '80s, he seemed like a perfect catch. At the time, she was in college and working odd jobs in Manhattan. He was a handsome surgical resident who spoke multiple languages, and had a passion for gourmet cooking, skiing and piloting small planes.
He was known for taking women on short flights for romantic dates. But his love of flying, and one flight in particular, was key for prosecutors to convince a jury that Bierenbaum was a murderer, someone who killed Katz in a homicidal rage and got away with it for 15 years.
"[Flying] is the crux of the story. I mean, it's the method that he tries to cover up his crime," said Steve Saracco, a former Manhattan assistant district attorney who prosecuted the Bierenbaum case with Bibb.
At first, their romance seemed "magical," said Katz's sister Alayne Katz, an attorney in New York state, but there were red flags. She said she went on a strange double date at a sushi restaurant, where Bierenbaum shoved food into her sister's mouth, then tried to do the same with her. Alayne Katz also said her sister called her one day, hysterically crying, because Bierenbaum tried to drown her cat in the toilet of their Upper East Side apartment.
"[She said], 'No, no, no, Alayne... we're gonna get rid of the cat and then everything's gonna be fine because he's gonna believe that I love him,'" Alayne Katz said. "And I'm like, 'No. Not really. You really have to get rid of Bob.'"
Even still, Gail Katz decided to go through with marrying him.
"My sister told me, 'I'm smart. I'm loving. My love will cure. This is going to work out,'" Katz said.
Over time, Gail Katz's friends and family members noticed the marriage was growing toxic. Bierenbaum was very controlling and at one point a neighbor said Katz told him she didn't feel comfortable at home.
One night, Alayne Katz said Bierenbaum caught his wife smoking on their balcony and he choked her into unconsciousness. Gail Katz reported the incident to a police administrative aide at a local precinct but nothing came of it.
"If this had happened in 2021 ... Robert Bierenbaum would have been in handcuffs immediately," Bibb said. "The fact that this was  ... nothing was done about it."
She convinced Bierenbaum to see a therapist, who ended up writing her a letter suggesting she should get out of the marriage and he was concerned he might kill her. Still, her sister said, Katz brushed it off for a while, and told her sister she was planning on leaving him.
Then on July 7, 1985, Gail Katz disappeared.
Bierenbaum told her family and police they had a fight at their apartment and Katz stormed off. He claimed to have stayed home. Her sister wasn't buying his story.
"She's not with me, and she's not with my parents, and at that moment I know that my sister's dead," Alayne Katz said. "And if she's not alive there's only one person who is a likely suspect to murder her, and it's Bob. There's no other suspect."
Days turned into weeks, then months, and there was still no sign of Gail Katz. Meanwhile, Bierenbaum was spending more time out in the Hamptons attending parties and dating other women.
Eventually, the Manhattan District Attorney's Office picked up the case and it went to chief investigator Det. Andy Rosenzweig.
Knowing of Bierenbaum's love of flying, Rosenzweig checked with the New Jersey airport Bierenbaum liked to rent planes from and discovered the doctor had taken a plane out on July 7, 1985, but Bierenbaum had doctored his flight log to make it look like he went flying the next day. Rosenzweig's working theory was that the doctor had killed his wife and thrown her body into the Atlantic Ocean.
However, without a body, the DA's office told Katz's family at the time they didn't have enough evidence to pursue charges against Bierenbaum. The DA's office closed their end of the investigation without bringing charges and the police department's investigation fizzled out. Bierenbaum continued to live his life as a free man.
Then in May 1989, nearly four years after Katz's disappearance, a torso washed up on Staten Island. This was a time before DNA forensics, and investigators tried to see if it belonged to Katz using old chest X-rays.
"An X-ray technician compared this X-ray with the torso and said, 'This is Gail,'" Alayne Katz said. "Now ... we have a body to bury. We have some closure."
By 1990, Bierenbaum had relocated to Las Vegas and opened his own plastic surgery practice. He started dating Dr. Stephanie Youngblood, a chiropractor, and she said their first year together was "perfect."
"We went to medical black tie events ... we went on a lot of ski trips," she said. "To go ski in Argentina in August when it's 114 in Las Vegas was absolute heaven."
Once again, he seemed like a perfect catch. Youngblood said Bierenbaum joined an organization that flew doctors and nurses in and out of Mexico to treat children who needed medical care.
But over time, cracks in Bierenbaum's facade started to appear. Youngblood said he eventually admitted he had been married before, but said his wife had disappeared and he didn't like to talk about it.
"The way he told the story, it was believable," Youngblood said. "He goes, 'Listen, this isn't for public knowledge. I would really appreciate it if you kept this under wraps.'"
But there were two incidents where Youngblood said Bierenbaum exploded with rage at her in front of other people. After the second time, she said she convinced Bierenbaum they should see a therapist, who recommended Youngblood end the relationship because her "life could be in danger with him." So she left him.
Bierenbaum started dating again and, in 1996, he married a Las Vegas gynecologist named Janet Challot. The couple moved to Minot, North Dakota, to start a new life together, and eventually had a baby girl.
No one in North Dakota knew about Bierenbaum's past, and in fact he had become somewhat of a local hero for saving a boy's life after he was bitten by a tiger at the North Dakota State Fair.
"He looked good, wore suits, loved his child, loved dogs, just all over all things that you think would be great characteristics," said Barb Cooper, who worked as a nanny for Bierenbaum.
Meanwhile, back in New York, Rosenzweig was nearing retirement and was looking into cold cases that still haunted him. The Gail Katz's disappearance case, which had been cold for 10 years by that point, was on his list.
In 1998, Rozenzweig got back in touch with her family. DNA forensics existed by then and he convinced the Katz family to exhume the torso they had buried years ago to have it tested.
"To our surprise, it came back that Gail Katz had been eliminated as a contributor of that sample," said Bibb.
When she heard the news, Alayne Katz said she was devastated.
"That little, little shred of closure that I had has now been ripped away," she said "I looked up at Dan [Bibb] and at Steve [Saracco], and I said, 'Now, you better get a conviction.'"
Investigators decided to re-interview everyone from the case. They flew to North Dakota to speak to Bierenbaum and Vegas to talk to Youngblood, as well as other women Bierenbaum was known to have dated.
One interview that struck investigators was with one of Bierenbaum's former girlfriends from when he was still in New York City. She told them she was with him one night when he got a phone call from Port Authority police in New York City, saying they thought someone had found Katz. He told them he would call them later, then told her, "I doubt it's Gail," according to investigators.
With this evidence, coupled with the proof Rozenzweig had uncovered years before that Bierenbaum took a flight on the day Katz disappeared, investigators moved forward with their case.
In 2000, a grand jury indicted Bierenbaum for second-degree murder. Fifteen years after her disappearance, he was finally going to trial for his wife's death.
"We knew it was going to be the toughest trial that we'd ever had. No forensics, no eye witnesses, entirely circumstantial," Bibb said. "There was no foregone conclusion to this case, by any stretch of the imagination."
One of the biggest hurdles at trial for prosecutors was to convince a jury Bierenbaum could fly a small plane and push a body out of it at the same time, so they filmed a recreation of this exact scenario to show it was possible.
"I thought the demonstration with the airplane was extremely effective, probably one of the critical moments of the case," said Leslie Crocker Snyder, the judge who presided over the case.
Bierenbaum's defense tried to paint Gail Katz as an unstable woman with mental health issues and a drug problem, but her sister said none of this was true. They called a witness, a retired textile manufacturer, who claimed he saw Katz in a bagel shop days after she disappeared, but prosecutors were able to easily point out the woman he described didn't match her description.
After just five-and-a-half hours of deliberation, a jury found Bierenbaum guilty of murder.
"I'll never forget ... squeezing my brother's hand and slamming it down on his thigh and saying, 'Guilty?' with a question mark at the end because, 'Did I just hear that?'" Alayne Katz said. "I was in shock ... I was relieved."
He was sentenced to 20 years to life in prison. He made attempts over the years to have his conviction overturned but was unsuccessful. Then, after serving 20 years of his sentence, he made his bombshell confession.
When ABC News showed the transcript of Bierenbaum's confession to Alayne Katz, she said, "This is exactly the same man that I knew 35 years ago." He hasn't changed ... he is incapable of a shred of remorse."
In honor of her sister, Alayne Katz said the nonprofit organization Pace Women's Justice Center was rebranded as Gail's House to bring greater awareness to domestic violence issues and provide resources to women. Katz said she allowed them to use her sister's name as a way to honor her memory.
"My sister's body has never been found ... Gail doesn't rest anywhere," Katz said. "Gail's House ... gave my sister a resting place. I feel my sister's spirit ... is here ... warning others ... inspiring others."
If you need help or need help supporting someone else, call the National Domestic Violence Hotline at 1-800-799-7233, or go to thehotline.org. All calls are toll-free and confidential. The hotline is available 24/7 in more than 170 languages.