With the core of his usual squads drawn from Leicester, Northampton, Sale and champions Saracens, Monday's group includes a mixture of established players, including veteran scrumhalf Danny Care, and those hoping to force their way in for the World Cup in France, that begins in September.
England play Wales twice, Ireland and Fiji in warm-up games in August.
"We are excited to get into camp and begin the first phase of our preparations,” Borthwick said in a statement.
"We have lots to get through before our first fixture in August against Wales in Cardiff, so the hard work starts now."
Henry Arundell, Ollie Hassell Collins, Will Joseph and Tom Pearson all requested to be listed as London Irish players despite the club going into liquidation and last week being suspended from all rugby.
Number eight Sam Simmonds ruled himself of contention for the World Cup in a recent conversation with Borthwick following his move from Exeter to Montpellier in France, while lock Joe Launchbury was not considered due to injury.
England 28-man training squad:
Forwards (15)
Jamie Blamire (Newcastle Falcons)
Alex Dombrandt (Harlequins) Tom Dunn (Bath Rugby)
Charlie Ewels (Bath Rugby) Ellis Genge (Bristol Bears)
Ted Hill (Bath Rugby)
Joe Marler (Harlequins)
Zach Mercer (Montpellier Herault Rugby)
Beno Obano (Bath Rugby)
Tom Pearson (London Irish)
Val Rapava-Ruskin (Gloucester Rugby) Kyle Sinckler (Bristol Bears)
Will Stuart (Bath Rugby)
Sam Underhill (Bath Rugby)
Jack Walker (Harlequins)
Backs (13)
Henry Arundell (London Irish) Danny Care (Harlequins)
Joe Cokanasiga (Bath Rugby)
Ollie Hassell-Collins (London Irish)
Will Joseph (London Irish)
Ollie Lawrence (Bath Rugby)
Joe Marchant (Harlequins)
Jonny May (Gloucester Rugby)
Cadan Murley (Harlequins)
Harry Randall (Bristol Bears) Henry Slade (Exeter Chiefs)
Marcus Smith (Harlequins)
Ben Spencer (Bath Rugby)