The aircraft, currently under certification, can hold 120 to 150 seats.
Bombardier said it would deliver CS300 planes to the Latvian flag carrier -- which has 13 of the aircraft on firm order and retains options for seven -- in the second half of 2016.
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Established in 1995, airBaltic serves about 60 destinations out of Riga, with a fleet of 25 aircraft.
"Bombardier's CS300 airliners are integral to the execution of airBaltic's business development strategy and fleet optimization plan," airBaltic chief executive officer Martin Gauss said in a statement.
"The CS300 aircraft... will offer airBaltic passengers a widebody, unparalleled in-flight passenger experience, perfectly complementing our modern fleet of 12 Bombardier Q400 turboprops."
Bombardier president Fred Cromer praised airBaltic as "one of Europe's leading innovative airlines."
"The unrivaled cabin living space offered by the CS300 aircraft will allow airBaltic to optimize its extra capacity seating offerings without comprising passenger comfort, together with world-class flying experience," he added.
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Bombardier has orders for 243 of its 603 C Series aircraft, which are facing significant delays after a 2013 maiden flight and cost overruns, with about Can$2 billion more than the Can$3.4 billion initially planned.
On Thursday, the French-speaking province of Quebec announced a US$1 billion investment in Bombardier to help the fledgling jetliner program get off the ground after the Canadian company posted a US$4.9 billion third quarter loss.
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