Jon McLaughlin, Barry Douglas, Dylan McGeouch, Scott McKenna and Oliver McBurnie also receive their first call-ups.
Manager Alex McLeish will also call up Hearts goalkeeper Jon McLaughlin and Aberdeen defender Scott McKenna.
Aberdeen players Scott McKenna, Kenny McLean and Ryan Christie have been named in new manager Alex McLeish's Scotland squad for the global friendlies against Costa Rica and Hungary later this month.
Costa Rica visit Hampden on 23 March, with another friendly in Hungary four days later.More news: Stock Jumping Abnormally High: Simon Property Group Inc. (SPG)
Returning boss Alex McLeish had no hesitation in selecting the Lancaster-born 21-year-old prospect in his pool of players.
The former Queens Park and Dundee United defender has enjoyed a nomadic career to this point, spending three seasons in Poland with Lech Poznan and a year in Turkey with Konyaspor before his move to the Midlands.
Perhaps Brendan Rodgers and McLeish have come to an agreement that means that Armstrong will be used sparingly in the games, but it does seem a bit of a gamble for a player who has spent so much time out injured to be selected for the national team.
McTominay has featured in each of United's last eight fixtures, making 19 senior appearances this season in total, and at times playing ahead of club record signing Paul Pogba.