Their 367-run partnership in a 425-1 total - the second highest in List A games - saw the Lions beat Sri Lanka A.
Duckett's 131-ball 220 not out was the ninth-highest List A score ever, while Bell-Drummond hit 171 off 139.
"Those guys are very capable of playing international cricket," said Flower, who now coaches the Lions.
It was a successful tri-series for Flower's team, as the Lions beat Pakistan A and Sri Lanka A twice apiece, with batsmen scoring more than 150 in each fixture.
Duckett scored 163 not out in a seven-wicket victory against Pakistan A on 19 July, before excelling again in Monday's 140-run win.
Dawid Malan - the 28-year-old Middlesex batsman who received his first England call-up for the Twenty20 squad against Sri Lanka in June - struck 185 in an 88-run Duckworth-Lewis win against Sri Lanka on 21 July.
Full international Sam Billings, who has played five ODIs and seven T20s, hit 175 in Sunday's second victory against Pakistan.
"I think English cricket will be better for these guys having to force their way in," Flower, who led the national team from 2009 to 2014, told the Tuffers and Vaughan Cricket Show on BBC Radio 5 live.
"These youngsters have to push their way in and earn their spot. It was an incredible series of innings and great partnerships, the selectors will almost certainly take interest in these results."
And the 48-year-old had special praise for Northants' 21-year-old opener Duckett, who he described as "a very talented young man".
"None of us can foresee the future but there's no reason why he can't go on and have a very successful international future," he said.

"I'm really looking forward to it - the home of Scottish football," said Rodgers ahead of his maiden visit.
"I hear the pitch is good, a nice big pitch suits the speed in our team and our intensity.
"The technical area goes right out to the end of the pitch, but you might need a taxi to get back to your staff."
This will be Rodgers' second taste of the Old Firm derby and his experience of the fixture got off to a great start with a 5-1 league victory at Celtic Park last month.
"It was a brilliant performance by the players in every aspect," he recalled. "Obviously this one is on a neutral ground, but we'll be looking to have a similar performance.
"We'll be prepared and focused. We know it's going to be a tough game. We anticipated that the last time."
Rodgers is also aware Celtic's visit to Hampden last season in the Scottish Cup semi-final against Rangers ended in defeat.
"The last experience there wasn't so good for Celtic," he said. "But it's a different team with a different mentality and we'll look to take that mentality into the game."
Rodgers lost two semi-finals in his time at Liverpool and is aiming to make it third time lucky at the club he joined in the summer.
"You have to perform - that's what's important and if you get little bit of luck along the way then hopefully you'll get the result that you want." said the Northern Irishman.
"So, for us, it's really looking at our performance level, which in the main has been at a really high level.
"My focus is always on my own team and, if we impose our style of play, it can give a lot of problems to opponents."
Unlike predecessor Ronny Deila, Rodgers would not be drawn on this semi-final being a step on the way to a potential domestic treble.
"It's very, very difficult to achieve," he said. "There's been great managers here in the past that haven't achieved that.
"But it's the first competition where we've a chance to win a trophy and I've always said that it's a priority for us because it's the first one.
"Our idea was to qualify for the Champions League, which we did, to make a good start in the league, which we've done, and then look to get to the League Cup final.
"Unless you can do that then you don't have the chance to win the trophies."
England Lions batsmen Ben Duckett and Daniel Bell-Drummond could play for England within a year, according to former national coach Andy Flower.
[NEXT_CONCEPT]
Manager Brendan Rodgers is sure Celtic can exploit the wide open spaces of Hampden when they meet Rangers in Sunday's League Cup semi-final.