Kinghorn, McCallum and Rae are all graduates of the BT Sport Academy.
The squad is also boosted by the return of several experienced players, absent from recent campaigns, including scrum-half Greig Laidlaw (ASM Clermont Auvergne), lock Richie Gray (Toulouse), centres Mark Bennett (Edinburgh Rugby) and Duncan Taylor (Saracens); and Worcester Warriors back-row David Denton.
Laidlaw, Gray and Bennett haven't featured for the dark blues since last year's championship, while it was 2015 when returning prop Jon Welsh last saw international action and longer still for veteran hooker Scott Lawson (2014).
A sequence of untimely injuries has meant Denton has been absent from test duty since the 2016 summer tour to Japan, while Taylor and London Irish prop Gordon Reid missed the 2017 autumn test campaign with a knee and head injury respectively.
Scotland Head Coach Gregor Townsend, said: "With the announcement of this 40-man squad, we are now entering the key stages of our preparation for this year's NatWest 6 Nations Championship.
"It's a bigger squad than we originally planned - which is largely down to the competition for places - with a number of players having performed well in the Autumn Tests and a few others putting their hands up for selection in recent weeks.
"We welcome Blair Kinghorn into our group for the first time, who has been in excellent form with Edinburgh this season, and also three of our most experienced players in Richie Gray, Greig Laidlaw and Scott Lawson.
"We are looking forward to working with them and the rest of the squad to ensure that we build on the momentum the players generated in 2017."
Mark Bennett (Edinburgh Rugby)
Alex Dunbar (Glasgow Warriors)
Nathan Fowles (Edinburgh Rugby)
Nick Grigg (Glasgow Warriors)
Chris Harris (Newcastle Falcons)
Stuart Hogg (Glasgow Warriors)
Peter Horne (Glasgow Warriors)
Ruaridh Jackson (Glasgow Warriors)
Huw Jones (Glasgow Warriors)
Lee Jones (Glasgow Warriors)
Blair Kinghorn (Edinburgh Rugby)
Greig Laidlaw (ASM Clermont Auvergne)
Sean Maitland (Saracens)
Byron McGuigan (Sale Sharks)
Ali Price (Glasgow Warriors)
Henry Pyrgos (Glasgow Warriors)
Finn Russell (Glasgow Warriors)
Tommy Seymour (Glasgow Warriors)
Duncan Taylor (Saracens)
John Barclay CAPTAIN (Scarlets)
Simon Berghan (Edinburgh Rugby)
Jamie Bhatti (Glasgow Warriors)
Magnus Bradbury (Edinburgh Rugby)
David Denton (Worcester Warriors)
Cornell Du Preez (Edinburgh Rugby)
Grant Gilchrist (Edinburgh Rugby)
Jonny Gray (Glasgow Warriors)
Richie Gray (Toulouse)
Luke Hamilton (Leicester Tigers)
Robert Harley (Glasgow Warriors)
Scott Lawson (Newcastle Falcons)
Murray McCallum (Edinburgh Rugby)
Stuart McInally (Edinburgh Rugby)
Gordon Reid (London Irish)
D'arcy Rae (Glasgow Warriors)
Ben Toolis (Edinburgh Rugby)
George Turner (Glasgow Warriors)
Hamish Watson (Edinburgh Rugby)
Jon Welsh (Newcastle Falcons)
Ryan Wilson (Glasgow Warriors)