LONDON, July 19 (Reuters) - Boeing said on Sunday it remains on track to deliver two new U.S. presidential aircraft in 2028, but explained that achieving this goal will require additional spending on a program already years behind schedule and billions of dollars over budget.

Boeing won a $3.9 billion contract in 2018 to build the two aircraft, but costs have since risen to more than $5 billion.

The two aircraft are intended to replace the current presidential aircraft, which entered service in 1990.

"We are on track (to deliver the two aircraft) in 2028," Boeing's Defense, Space & Security chief Steve Parker told reporters ahead of the Farnborough Airshow in Britain, adding that he expects the first aircraft to begin testing next year.

He added, "I expect we will see some increase in costs there, as we progress with the work and complete the connections and structures, as well as finalize our certifications."

In May 2025, the United States accepted a luxury Boeing 747 from Qatar for use as a temporary presidential aircraft.

Since then, the aircraft has been in service on a temporary basis. US President Donald Trump decided against returning from Turkey on the Qatar Airways plane due to security concerns, opting instead to return on an older presidential aircraft.