Do you think Martins and Gibsons from that era sound comparable? Because the 60's ones, that I'm more familiar with, certainly don't. There's a jangle to a good 60's Gibson I've never hear from any Martin, but my knowledge of prewar guitars is very limited.
Not in the least. Well, at least not beyond "These are both basically flat-top acoustic guitars." They have about as much in common to me, sonically, as a Telecaster and an ES-335.
You mention 60s Gibson acoustics, but those are another matter, entirely. Not even part of the same conversation, really. Unlike Martins, Gibson flat tops underwent a series of radical shifts (got WAY worse, on mutiple fronts, to me) over the course of the 1950s.
If you know or have played a few postwar Martin D-28s, especially at least one or two from before 1969*, then you have some idea of what an old Martin basically "is." It's just that the older ones sometimes (not always) have a little extra 'magic.'
With Gibsons, until you have played a few pre-1952 (or so) J45, J35, Jumbo, Advanced Jumbo, or Southern Jumbo, you might be missing a whole 'flavor' of flat top acoustic**.
Words always fail and are almost useless in their imprecision, but compared to a Martin, the Gibson slope-shouldered dreads (especially the "banner" guitars) as a population may be describable as throatier, more resonant, darker and less sparkly, with an uncommon sort of enveloping 3D sound on the better examples. Subjectively "warmer" and "thicker" sounding than a Martin (which isn't always desired, of course!) Better examples could be described (cringe) as "round" "warm" "resonant" "rich" "luxurious" "buttery." Worse examples sometimes approach "dull" "tubby" "flat" though as a population, such bad apples are relatively rare in the wartime era. Most are good ones, and a good one will vibrate itself out of your lap, all over the guitar just resonating like crazy. The "jangle" you mention of the 60s Gibsons is not generally found, to my ear.
With a Martin D, on the other hand, I'd describe a good one as "cutting" "tight" "powerful" "bright" "silvery" "shimmery" "glossy" "sparkling" etc. A bad one might be "thin" or "nasal" or "brittle" or have wolf tones (again here, with the pre-war guitars the "bad" ones are somewhat rarer). Even the good ones don't seem to vibrate and shake as much as a Gibson-- you get the sense that it's a better-engineered guitar in terms of getting volume, projection, and "cut." On a typical example the top may be VERY alive, but that energy is more focused on the top, where on many Gibsons it's often more diffuse.
A Gibson slope-shoudlered dreadnaught isn't the best for bluegrass... a D28 will be the pick there, for absolute sure. The Martin is also great when the acoustic is functioning in a percussive way in the track, like a "shaker."
But for many, many other things, especially solo acoustic-and-vocal things, the Gibson is great. I'd like to have a great Martin just because, but for most of what I do I gravitate toward what the Gibsons do.
*1969 is when Martin switched from Brazilian to Indian rosewood
**As cool looking as the big, fancy, mustache-bridge J200 etc are, they bear no sonic relation to the slope-shouldered dreadnaught ("jumbo") guitars, and are a separate conversation.