Airlines are poised for their busiest year of pilot hiring in more than three decades as the industry tries to restock a workforce reduced during the pandemic and strained by a quick rebound in travel.

Travelers returned in force this year, and the boom is expected to continue as international borders reopen and corporations send workers back out on the road. After seeking to conserve cash by urging thousands of pilots to retire early, airlines are now on an unparalleled hiring spree.

Major...

Airlines are poised for their busiest year of pilot hiring in more than three decades as the industry tries to restock a workforce reduced during the pandemic and strained by a quick rebound in travel.

Travelers returned in force this year, and the boom is expected to continue as international borders reopen and corporations send workers back out on the road. After seeking to conserve cash by urging thousands of pilots to retire early, airlines are now on an unparalleled hiring spree.

Major U.S. carriers are on track to hire around 4,200 pilots this year and more than 9,000 next year according to FAPA.aero, a Nevada-based career and financial adviser for professional pilots. That would be the busiest year for pilot hiring in more than three decades, according to FAPA’s figures. In 2019, when airlines were hiring at a rapid clip, major U.S. carriers hired about 5,000 pilots.

Regional airlines, where many pilots start their careers, are also on the prowl, competing to offer rich bonuses to lure new recruits.

“There are not enough pilots out there right now to go around,” said Tim Genc, chief adviser at FAPA.

Before the Covid-19 pandemic hit, Eric Bensinger was getting his flight log books in order and preparing cover letters to nab a job as an airline pilot.

Then airlines stopped hiring. Pilot training programs were suspended, young pilots without seniority faced furloughs, and older ones considered whether to cut their careers short.

“I was getting a little concerned,” Mr. Bensinger said. “I started looking at other industries.”

Over the summer, he had three job interviews and three offers, and accepted a job that offered an initial $10,000 bonus when he starts training and an additional $5,000 after a year. “I still can’t believe it right now,” Mr. Bensinger said.

Airlines have been plagued by staffing snarls as they have emerged from their pandemic-induced pullback. Workers from baggage handlers to fuel-truck drivers have been hard to find. Retraining pilots who were out on leave or had to switch to new aircraft types has created logjams. While those shortfalls have contributed to dayslong disruptions that have upended thousands of flights, airlines say such problems are likely to be short-lived.

Large airlines say they aren’t having trouble finding candidates to fill out their pilot rosters now, but analysts say the pandemic might have sped up a long-feared crunch.

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Worries about finding the next generation of pilots have dogged the industry for years. The flow from the military—long a source for commercial airlines—has slowed. A 2013 federal rule mandating that aspiring pilots fly 1,500 hours before being hired at a regional carrier added years and tens of thousands of dollars of upfront costs. Major airlines have formed partnerships with flight schools and universities in an effort to ensure a steady pipeline of candidates.

Pilots are also getting older, with a greater share approaching the mandatory retirement age of 65. A long-anticipated wave of exits accelerated during the pandemic as airlines offered buyouts and retirement packages to conserve cash. At the four largest U.S. carriers, more than 4,400 pilots opted to depart last year.

Now major carriers are pulling pilots up from regional affiliates to backfill, and those smaller carriers are trying to catch up again after a yearlong pause in hiring, said

Geoff Murray, a partner at consulting firm Oliver Wyman.

“There is already a shortage,” he said.

Mesa Air Group Inc., a Phoenix-based carrier that flies for American Airlines Group Inc. and United Airlines Holdings Inc., is offering new hires a $20,000 signing bonus. Jonathan Ornstein,

Mesa’s chief executive, said it is getting harder to hold on to pilots as major airlines and freight carriers vie for them: Mesa’s attrition rates have tripled.

“Everybody is hiring,” he said.

Mr. Ornstein said he believes barriers are too high for people trying to enter the industry.

“We’re looking at a reservoir that’s losing water,” he said.

The first signs of a pinch are emerging. At Horizon Air, a regional carrier owned by Alaska Air Group Inc., “aggressive pilot hiring across the industry is expected to lead to abnormal attrition levels,” Alaska Chief Financial Officer Shane Tackett said last month. A spokesman for Alaska said Friday that the airline has a healthy pipeline of applicants and has taken steps to backfill Horizon pilots who have moved up to the main line or another carrier.

“‘Everybody is hiring.’”

— Jonathan Ornstein, chief executive, Mesa Air Group

American Airlines executives said last month that they haven’t been able to rely as heavily on some regional airlines because of pilot shortfalls that have constrained the smaller carriers’ ability to fly their schedules.

“We’re probably not flying as much regional as we would have flown,” American President Robert Isom said during an earnings conference call, adding that the issue will be resolved over time. “This will be brought into balance just simply based on economics. People will want to come into the profession.”

American started hiring pilots again in July and expects to have brought on 440 by the end of this year. Next year it plans to hire more than 2,000. Chip Long, vice president of flight operations at American, said hitting those numbers shouldn’t be a problem.

“Do I wish there were hundreds of thousands of more pilots? Sure. But we’re very competitive,” he said.

The three regional carriers owned by American recently started offering a package that can add as much as $150,000 in upgrade and retention bonuses. The goal is to prevent poaching, said Ric Wilson, vice president of flight operations for Envoy Air Inc., one of American’s wholly owned regional carriers.

“I think it’s only natural that some of these other airlines…that they would want our guys to fill their cockpits. We want to make sure it doesn’t happen,” he said.

Mr. Wilson said he hasn’t had problems finding candidates. Envoy is on track to hire 240 pilots this year, with about four times as many qualified applicants as the carrier has open positions.

Pilots have been whipsawed by how quickly their fortunes have shifted over the past 18 months. Liz Rahmlow, who was building her hours by working as a flight instructor when the pandemic hit, said it was unclear even earlier this year when—or if—she would be able to find a job at an airline. She is now about a month into her training at a regional airline.

“Personally, I’m really positive right now,” she said. “It’s been a very, very fast recovery.”

Write to Alison Sider at alison.sider@wsj.com