That includes stress testing a trial of "a major pharmaceutical company’s thermal packaging," CEO Doug Parker said in a LinkedIn post earlier this week.
The flight did not involve any vaccine doses, a spokesperson confirmed to ABC News.
Parker said the airline "successfully moved" the thermal packaging of the company, which was not identified, last week.
Pfizer and BioNTech were the first to submit a request to the U.S. Food and Drug Administration for emergency use authorization of their COVID-19 vaccine candidate. Distribution of their vaccine poses a potential challenge: It must be kept at around minus 94 degrees Fahrenheit -- far colder than a typical freezer can accommodate.
Because of these temperature requirements, the company will ship vaccine doses in special temperature-controlled thermal boxes packed with dry ice.
American Airlines said in a statement to ABC News that it has established a "network of team members that specialize in temperature-critical shipments," and that it will work with the Federal Aviation Administration on "regulations governing shipments transported with dry ice."
Utilizing cargo and commercial carriers will allow the vaccine to be "distributed as quickly as possible," Parker said.
United Airlines has also been preparing to "support a vaccine distribution effort on a global scale," a spokesperson said in a statement to ABC News.
The company is ready to "safely and effectively support" the vaccine transportation needs of its pharmaceutical customers, the spokesperson said.
The Wall Street Journal reported on Friday that the airline has started flying Pfizer's COVID-19 vaccine.
Pfizer and BioNTech said they will be ready to distribute the vaccine within hours after authorization.
Representatives of the companies are set to meet with the FDA on Dec. 10, after which an independent advisory board will make a recommendation on authorization.
Moderna's candidate will also likely be among the first vaccines distributed. Earlier this month, the company announced that initial phase 3 data showed its vaccine is up to 94.5% effective.
Gen. Gustave Perna of the Trump administration's Operation Warp Speed vaccine program told ABC News this week that vaccines will be "on the street" in communities just 24 hours after being authorized by the FDA.
Elderly care facilities and health care providers will be the first offered the vaccine, according to U.S. officials.
ABC News' Amanda Maile contributed to this report.