Both managers handed game time to some of their promising talent with West Brom goalkeeper Luke Daniels making just his second start while Arsene Wenger started Ryo Miyaichi, Serge Gnabry, Isaac Hayden and Thomas Eisfeld. He also introduced Chuba Akpom, Kristoffer Olsson and Hector Bellerin off the bench.
The Gunners edged ahead just after the hour-mark when 20-year old midfielder Eisfeld scored in his first start for the club before Baggies starlet Saido Berahino responded with just under 20 minutes remaining.
The match went all the way to the end of extra-time with neither side able to break the deadlock before the Gunners saluted 4-3 on penalties with the likes of 17-year old Akpom and 18-year old Olsson netting from the spot, before Nacho Monreal finished it off, delighting Wenger.
"I am proud. We played with many young players and they showed they are not only good footballers but that they can fight as well," he said.
"I decided to go for young players with the penalties and they coped well with the pressure.
"Of course, penalties are a bit of a lottery, but overall I think the most important thing is that we played well in the game and that the young players came out and showed that they can play the way we want to.
"It is always good to know that you can win even when you thought you were dead and tired.
"If you dig deep and in the end you are rewarded, that is a very important lesson for a young player.
"We kept going forward and trying to win the game until the end and that is very positive."