The deal for the MRJ90 jets -- Japan's first domestically-made commercial aircraft for about half a century -- is a major boost for Mitsubishi.
In December it postponed delivery of the plane, a little more than a month after the maiden test flight.
Mitsubishi and Miami-based Aerolease Aviation LLC signed a letter of intent at the Singapore Airshow for 10 firm orders and 10 options for the MRJ90 planes to be delivered in 2018.
Mitsubishi Aircraft Corp president Hiromichi Morimoto said the order "has a significant importance in the MRJ programme".
Both companies expect to sign a definitive agreement soon for the orders, which would be worth around $940 million at list prices.
Excluding the deal with Aerolease, Mitsubishi had a total of 407 orders for its jets, comprising 223 firm deals and 184 options.
Its clients include Japan Airlines, Air Nippon Airways and Air Mandalay.
Jep Thornton, a partner for Aerolease, said that despite Mitsubishi's move to postpone delivery of the plane to the second quarter of 2018, he remained confident about the capabilities of the Japanese firm.
Teething problems were common in new aircraft launches, he said.
In December Mitsubishi cited "several issues" discovered after the November flight for delaying shipments by one year.
The plane's system software had to be upgraded and its fuselage strengthened, it said at that time.
The twin-engine jet's November flight came about half a century after the last Japanese-made commercial plane took to the skies.
The YS-11 turboprop which made its debut in 1962 was discontinued about a decade later.
Japan -- and its MRJ jet -- are competing with other regional passenger jet manufacturers such as Brazil's Embraer and Canada's Bombardier.
Mitsubishi unveiled the jet -- which is 35 metres (115 feet) long and seats about 80 passengers -- in October 2014.
It had previously planned to make the first delivery to Japan's All Nippon Airways in 2017.
The short-to-medium-haul plane was backed by the Japanese government and a consortium of major firms including Toyota.
Automaker Honda is also developing a small private jet in the United States, which was unveiled in Japan earlier this year.
