How’s this for an MVP? Wikipedia has:
Bill Gates was a student at Harvard University and Paul Allen worked for Honeywell in Boston when they saw the Altair computer on the cover of Popular Electronics. They had previously written software for the earlier Intel 8008 microprocessor and knew the Intel 8080 was powerful enough to support a BASIC interpreter.
They sent a letter to MITS claiming to have a BASIC interpreter for the 8080 microprocessor.
Roberts was interested, so Gates and Allen began work on the software.
Wikipedia – Ed Roberts (Computer Engineer)
The Smithsonian Institution has more:
… a major milestone for us was when we were walking through Harvard Square, one time, and saw this Popular Electronics magazine. And it was kind of in a way, good news and bad news. Here was someone making a computer around this chip in exactly the way that [Microsoft co-founder] Paul [Allen] had talked to me, and we’d thought about what kind of software could be done for it, and it was happening without us. And for all we knew maybe they had some software people, they were just going to go charge off and do this thing.
So, we wrote this company immediately. Sent them a letter. We have a copy of that somewhere. But anyway, a letter on Traf-O-Data’s stationary, because it was the only stationery we had. We offered to do a BASIC for them. And they thought that was interesting. They called back and said, “Well, you’re serious? We have a lot of strange people calling us.”
…
I had no doubt that I could write a BASIC Interpreter. I thought through in my head all the things that I hadn’t done before on those mini-computers. Doing things really small, fascinated me. …
I had some ideas on how to do things a little bit in a new way. There was no doubt in my mind we could write a BASIC. I was fairly self-confident in those days. We didn’t know how long it would take us. And it was kind of funny because we were sort of acting like we had it already. We went to work day and night.
…
So, we wrote without ever seeing this machine [pats the Altair], except in this picture, and the simulator and got the BASIC running. And then we called them back and said, “By the way, what’s the sub-routines for reading a character from the Teletype and writing to the Teletype — how do you do that?” And we got Bill Yates on the phone, who was the co-author of this article. And he said, “Well, that is pretty interesting. All these other guys call us up and say they are going to do things. But nobody ever asked us how you get data in and out of the machine. You guys sound pretty serious. You ought to come out and show it to us.” And, because we’d never had the chip, just the book from Intel, if we had made any mistake in terms of how the instructions worked, the thing never would have run.
And so Paul was scheduled to fly out to Albuquerque. He decided to go get some sleep. I stayed up all night reading the book to see if we’d miscoded some of the instructions. And finally, decided it was all okay, punch out the paper tape, and made sure Paul got that before he went off on his plane…
He took the BASIC to MITS. They had a machine they had run with 6K of memory, which for them was a big, big thing. And loaded up the paper tape. The first time, for some reason it didn’t work. The second time they loaded it in and it worked.
…
Anyway, it came up, it said Ready, and he typed in a program, Print 2 + 2, and it worked. He had it print out squares, sums and things like that. He and Roberts, the head of this company, sat there and they were amazed that this thing worked. Paul was amazed that our part had worked, and Ed was amazed that his hardware worked, and here it was doing something even useful. And Paul called me up and it was very, very exciting.
Bill Gates – interview with The National Museum of American History – Smithsonian Institution