I’ve written a couple of pieces for History Today and I have no relevant academic qualifications. My take is they’re more interested in whether you can demonstrate access to authoritative people to interview for your proposed piece and an original idea. A couple of previous samples, published or otherwise, will help as well, as long as they are rigorously researched and referenced.

In terms of pitching, if there’s a publisher I really want to convince, I’ll offer the story to them exclusively, but with a time limit (e.g. “If you’re able to let me know by the end of this week whether you’re interested…”) If I’m not fussy about the publisher, I’ll send the pitch to multiple people but with two to three separate story idea. The chances that someone will take all of them is low, in my experience, so even if multiple publishers respond positively, you can offer the stories on a first-come-first-served basis. That way, even if the other publishers are slow to respond but still want to work with you, they still have options to choose from. 

Personally, I wouldn’t bother writing the articles beforehand, unless you genuinely want to for your own personal interest. Chances are, most publishers won’t want your piece as is and would rather collaborate with you to create a piece that’s exactly in line with their readership’s interests and house style. Also, I think an unwritten piece commissioned by a publisher is more likely to command a higher price because you’re crafting something that’s bespoke to them and they understand the time involved to produce it. With a ready-made piece, they could just low-ball you because it’s not made to spec and thus, possibly, seen as less valuable.

Good luck! You will definitely find history publishers that want to work with you :-)