A stunning second-half performance ensured United progressed to the next round of the U21 Premier League Cup on Monday night.
The Blades trailed Birmingham 1-0 at half-time at Solihull Moors' Autotech Stadium, after Blues striker Denny Johnstone had given converted Mitch Hancox's deep cross, but two Otis Khan penalties and another from trialist Scott Fenwick completed a deserved turnaround for Chris Morgan's side.
Another major positive for the Blades was the return to action of defender Terry Kennedy, who has been sidelined since the beginning of pre-season, with a knee problem.
The home side enjoyed the lion share of possession in the first-half and they were rewarded in the 24th minute when Hancox delivered an inviting centre that was converted at the back post by the onrushing JOHNSTONE.
But having rarely threatened in the final third in the opening 45 minutes, United restored parity before the hour mark. Packwood brought down KHAN in the penalty area, allowing United's number 7 to dust himself down before converting from the resulting spot-kick.
Less than 60 seconds later the Blades went in front, a high ball from the right saw FENWICK challenge Preston in the Blues goal and it was the trialist striker that got the better of the battle to score from close range.
United continued to pile on the pressure with Calvert-Lewin and Cockerline both denied by the woodwork, but soon it was 3-1 with KHAN netting his second penalty of the night, after a combination of Preston and Hancox had been adjudged to have impeded Fenwick.
The Blades saw the game out well, although Whiteman had to produce a timely block to deny Lee before Johnstone's header clipped the outside of the post from another Hancox cross on the left.
Blades: Willis, Paling, Scarisbrick, Whiteman, Evans, Banton, Khan, Calvert-Lewin, Cockerline (Wright 65), Fenwick (Hinchcliffe 86) Hamilton (Kennedy 72).
Unused: Eastwood, Cheeseman.