FARNBOROUGH, England (Reuters) - Brazilian planemaker Embraer ( ERJ ) on Monday signed the contract to formalize the sale of nine C-390 Millennium military aircraft to the Netherlands and Austria, further expanding its defense order backlog.

The two European countries had previously selected the aircraft to replace their aging fleets of Lockheed Martin's ( LMT ) Hercules C-130s, but a final deal with both nations placing a joint order was still pending.

The firm order announced at the Farnborough Airshow includes five aircraft to the Netherlands and four to Austria, with deliveries expected to commence in 2027 and conclude by 2030, according to the Brazilian firm.

"It is one of the most important days in the history of Embraer Defense & Security as it represents our largest contract for C-390 exports," Embraer's ( ERJ ) defense head Bosco da Costa Junior told Reuters.

Expanding its presence abroad with more C-390 sales has been a key goal of Embraer's ( ERJ ) defense division, including selling the military transportation airplane to NATO members such as the Netherlands.

In addition to the two European countries and Embraer's ( ERJ ) home country Brazil, nations such as Portugal, Hungary, the Czech Republic and South Korea have also tapped the aircraft for their fleets.

Austria and the Netherlands "had already selected the C-390 as the platform meeting all their requirements but now that becomes a purchase commitment, with deliveries and the aircraft being added to our production line," Costa Junior said.

The company did not disclose the deal's price tag.

Embraer ( ERJ ) is currently working to increase output of the tactical airplane. Its production line was planned for 18 aircraft per year, but this year four planes are being delivered and next year another six, according to Costa Junior.

"We forecast (annual) production to reach 12 aircraft by 2030," the executive said. "The challenge right now is for supply chain to keep up with it. A few months ago we had a 'C-390 Day' with our main suppliers to make sure this ramp up will happen."

Embraer's ( ERJ ) defense backlog stood at $2.1 billion at the end of the second quarter, when it delivered one C-390 aircraft.

"This sale makes our backlog very NATO-concentrated, but we also had a sale last year to South Korea. Asia, Middle East and obviously Europe are regions where the C-390 has been standing out," Costa Junior said. "We are very optimistic about future purchases by other countries."

India, Saudi Arabia and the United States are seen as strategic markets for Embraer's ( ERJ ) defense unit, with Sweden also a potential customer.