May made her return for Gloucester on Saturday, presenting as a second-half substitute after five months out recovering from a knee injury.
Exeter star Nowell, who broke his arm during the Guinness World Cup in England Six Nations defeat against France in March, also returned for his club during the final round of a regular season Gallagher Premiership ago.
Premiership semifinal players Leicester, Saracens, Harlequins and Northampton were not considered for election.
England head coach Eddie Jones said: “With some players unavailable due to the Premiership semi-final, we have the opportunity to call in a number of new players.
“It’s a great opportunity to learn about them and see what potential they have to play a role in the Australia tour.
“We look forward to continuing the good work of our last camp to develop this team on and off the field.”
May missed the entire Six Nations campaign this season, as did an Exeter lock Jonny Hillwhich is also included.
Hill has not played since early January due to a tight fracture of his lower leg.
And with the Premiership finals and final yet to come, he will keep his fingers crossed for no further setbacks.
England is ready to host the home Barbarians in Twickenham on Sunday, June 19 before their Down Under tour in July.
The side will travel to Australia to face the Wallabies in Perth on Saturday 2 July, before heading to Brisbane on Saturday, 9 July, and finally, Sydney where they will play their final game on Saturday, 16 July. All start times are 11:05 British time.
Biyi Alo (Wasps, hatched)
Alfie Barbeary (Wasps, hatless)
Jamie Blamire (Newcastle Falcons, 6 caps)
Callum Chick (Newcastle Falcons, 2 caps)
Freddie Clarke (Gloucester Rugby, headless)
Tom Curry (Sale Sharks, 40 caps)
Trevor Davison (Newcastle Falcons, 2 caps)
Charlie Ewels (Bath Rugby, 30 caps)
Will Goodrick-Clarke (London Irish, headless)
Jonny Hill (Exeter Chiefs, 12 caps)
Ted Hill (Worcester Warriors, 2 caps)
Sam Jeffries (Bristol Bears, headless)
George McGuigan (Newcastle Falcons, headless)
Tom Pearson (London Irish, headless)
Bevan Rodd (Sales Sharks, 2 caps)
Patrick Schickerling (Exeter Chiefs, headless)
Jack Singleton (Gloucester Rugby, 3 caps)
Will Stuart (Bath Rugby, 20 caps)
Sam Underhill (Bath Rugby, 28 caps)
Jack Willis (Wasps, 3 caps)
Henry Arundell (London Irish, headless)
Charlie Atkinson (Wasps, hatless)
Mark Atkinson (Gloucester Rugby, 1 cap)
Orlando Bailey (Bath Rugby, headless)
Joe Cokanasiga (Bath Rugby, 11 caps)
Ollie Hassell-Collins (London Irish, headless)
Will Joseph (London Irish, headless)
Jonny May (Gloucester Rugby, 69 caps)
Jack Nowell (Exeter Chiefs, 39 caps)
Paolo Odogwu (Wasps, hatless)
Max Ojomoh (Bath Rugby, headless)
Tom Parton (London Irish, headless)
Adam Radwan (Newcastle Falcons, 2 caps)
Harry Randall (Bristol Bears, 6 caps)
Dan Robson (Wasps, 14 caps)
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