Has a kit version of the Marin MB-2 bomber ever been available ?

Yes. Originally a 50s/60s vintage kit was boxed by ITC & UPC. It was most recently reissued by Glenco models.

Full details:

https://www.scalemates.com/topics/topic.php?id=3322

crackers

First produced in June 1920, the Glenco Marin MB-2 bomber was the first designed American bomber...

Actually the MB-2, as its name should reveal, was the second bomber designed for the Army Air Service. The first was the original Glenn Martin Bomber of 1918, which was also the first aircraft designed by Martin's new engineer, Donald Douglas.

For what it's worth. Here's my rendition of that model.

It's not bad, but there are some nasty ejector pin marks and very thick flash.

You did good with that one.  I've only seen one built in all these years.  I don't know if it's the subject as compared to more modern aircraft or the quality of the kit that keeps it from being seen more.  I tried making one last year and lost a part, and had trouble with the way other parts came out.  I would like to try one again some day.

Looks good!

I once made my own drawings of the original GMB in hopes of building a model. Sadly my drawings are all lost now, as is my drafting table and tools. No room where I live.

That reminds me I was going to build a Army biplane and put it in modern cammo and enter it in a show and see what kind of comments I get.