Part 1/2:

Eric Clapton had become closer friends with George Harrison and Pattie Boyd around the late 1960s; thanks to their friendship, Clapton had the rare honour of playing on a Beatles record who wasn't in the band (on Harrison's White Album song 'While My Guitar Gently Weeps'). And it's fair to say that the love triangle between Clapton, Harrison, and Boyd was...considerably more complex than a triangle.

Pattie Boyd was a model and actress who had met George Harrison on the set of A Hard Day's Night. According to Boyd, in her autobiography Wonderful Tonight: George Harrison, Eric Clapton, And Me, by the point in 1968 (when Boyd was 24 and Harrison was 25) when the Beatles went to India to study with the Maharishi, Pattie and George were not in a good place in their relationship. George seemed to be depressed and withdrawn, and he was not faithful. According to Boyd:

And there were other women. That really hurt. In India George had become fascinated by the god Krishna, who was always surrounded by young maidens, and came back wanting to be some kind of Krishna figure, a spiritual being with lots of concubines. He actually said so.

And no woman was out of bounds. I was friendly with a French girl who was going out with Eric Clapton. She was always flirtatious with George, but so were a lot of girls and he, of course, loved it. Then she and Eric broke up—Eric told her to leave—and she came to stay with us at Kinfauns [the Harrisons' house in Esher, Surrey].

...the French girl didn’t seem remotely upset about Eric and was uncomfortably close to George. Something was going on between them, and I questioned George. He told me my imagination was running away with me, I was paranoid. Soon I couldn’t stand it so I went to London to stay with Belinda and Jean-Claude. Six days later George phoned me to say that the girl had gone and I went home. I was shocked that George could do such a thing to me.

Eric Clapton and George Harrison became close friends over the course of 1968 and 1969, and Clapton became infatuated with Pattie. According to Boyd's autobiography:

He and George had become close friends; they played, wrote music, and recorded together. At that time his girlfriend was a model called Charlotte, but I was aware that he found me attractive—and I enjoyed the attention he paid me. It was hard not to be flattered when I caught him staring at me or when he chose to sit beside me or complimented me on what I was wearing or the food I had made, or when he said things he knew would make me laugh or engaged me in conversation. Those were all things that George no longer did.

According to Clapton:

I remember thinking that her beauty was also internal. It wasn’t just the way she looked, although she was definitely the most beautiful woman I had ever seen. It was deeper. It came from within her, too. It was just the way she was, and that captivated me. I had never met a woman who was so complete, and I was overwhelmed. I realized that I would have to stop seeing her and George, or give in to my emotions and tell her how I felt.

As Eric Clapton's band Cream broke up and he was in the process of forming his new band Blind Faith with Steve Winwood, he claims to have met a sixteen-year-old upper class girl named Alice Ormsby-Gore, who clearly played a role in genesis of 'Layla':

I found her completely compelling. With her wistful quality and the Arab clothes she used to dress in, she was straight out of a fairy story. This fantasy was encouraged by Ian Dallas, who told me the tale of Layla and Manjun [sic], a romantic Persian love story in which a young man, Manjun, falls passionately in love with the beautiful Layla, but is forbidden by her father to marry her and goes crazy with desire. Ian was forever saying that Alice was the perfect Layla, and while he thought Steve should be her Manjun, I had other ideas.

This relationship with Ormsby-Gore was cut short by an odd incident after a gig George had played with Delaney & Bonnie in December 1969:

Pattie had showed up at the show, accompanied by her younger [seventeen year old] sister Paula. After the show, when we were all back at the hotel, George, who was motivated just as much by the flesh as he was by the spirit, had taken me aside and suggested that I should spend the night with Pattie so that he could sleep with Paula. The suggestion didn’t shock me, because the prevailing morality of the time was that you just went for whatever you could get, but at the last moment, he lost his nerve and nothing happened. The end result was not the one George wanted, as I ended up spending the night with Paula instead of him.

However, Boyd doesn't seem to have realised she was to be used as a bargaining chip so that George could sleep with her sister:

Then, to add to [Boyd's mother's] worries, that night in Liverpool Eric fell for Paula. After the show we all went to a restaurant and everyone was quite drunk and raucous. When the rest of us went back to the hotel, we left Eric and Paula dancing. George was furious with him. He was protective of Paula.

This incident ended Clapton's relationship with Ormsby-Gore:

...In Alice’s absence, Paula, a surrogate Pattie, moved into Hurtwood [Clapton's house nearish to Kinfauns, in Ewhurst, Surrey], where I was almost immediately involved in setting up another band [Derek And The Dominos]. It was a stopgap relationship, and I think we both knew that, but she reminded me a lot of Pattie, and for the moment I had no qualms about that.

However, Boyd's autobiography suggests that Paula saw their relationship quite differently, and was devastated when it ended:

She had been seriously in love with Eric, but he destroyed her pride, her self-esteem, and her confidence—which were already fragile. On top of that her big sister, traditionally the nurturer in the Boyd family, was the last person to whom she could turn for comfort.

And then it finally happened. According to Clapton's autobiography:

One night I called up Pattie and told her “the truth,” that it was not Paula I was interested in, or any other girl she might see me with, but that she was the one I really wanted. In spite of her protests that she was married to George and that what I was suggesting was impossible, she agreed to my coming over to talk to her. I drove over there, and we talked about it over a bottle of red wine and ended up kissing, and I sensed for the first time that there was some kind of hope for me. I knew then what I had suspected for some time, that all was not well in her marriage.

Clapton and Boyd had clandestine meetings, and at one, Clapton played 'Layla' for her for the first time:

Another of our secret meetings took place in London one afternoon...Eric took me there because he wanted me to listen to a song he had written. He switched on the tape machine, turned up the volume, and played me the most powerful, moving song I had ever heard.

It was “Layla”—about a man who falls hopelessly in love with a woman who loves him but is unavailable. He had read the story in a book he had been given by a mutual friend, Ian Dallas. Ian had given me a copy too. It was called The Story of Layla and Majnun by the Persian writer Nizami. Eric had identified with Majnun, and was determined that I should know how he felt. He had written the song at home and recorded it in Miami with the Dominos.

And then, according to Clapton:

A little while later I saw George at a party at Stigwood’s house and blurted the whole thing out to him: “I’m in love with your wife.” The ensuing conversation bordered on the absurd. Although I think he was deeply hurt—I could see it in his eyes—he preferred to make light of it, almost turning it all into a Monty Python situation. I think he was relieved in some way, though, because I’m sure all the time he knew what had been going on, and now I was finally owning up to it.

Boyd found that encounter considerably more mortifying:

It was a great party, and I felt elated by what had happened earlier in the day, but also deeply guilty.

Much later in the evening, George appeared. He was morose, and his mood was not improved by walking into a party that had been going on for several hours, and most of the people there were out of it. He didn’t want to speak to anyone, just find me. He kept asking, “Where’s Pattie?” but no one seemed to know. He was about to leave when he spotted me in the garden with Eric. It was early morning, just getting light, and very misty. He came over to us and said, “What’s going on?”

To my complete horror, Eric said, “I have to tell you, man, that I’m in love with your wife.”

I wanted to die.

And I said, “George, I’m coming home.”

I followed him to his car, we got into it, and he sped off. When we got home I went to bed and he disappeared into his recording studio.

This event likely occurred in July 1970 (Boyd mentions the party occurring after going to see Oh! Calcutta!, which debuted in London in July 1970, and she was in the kind of circles that would have gone to the debut). The original studio version of 'Layla' was recorded in September 1970, in Miami, for the Derek And The Dominos album released in November 1970. George Harrison was heavily involved in the formation of that band, who basically first came together as the backing band on his album All Things Must Pass. Harrison was also backstage for their first concert in June, and involved in naming the group. However, perhaps given the events of July 1970, Harrison wasn't involved in the sessions in Miami, and the band don't seem to have played the song live at that first concert. Boyd claims in her biography that she only saw Clapton once between July 1970 and his slide into addiction at the end of November 1970. So it seems likely that George Harrison didn't hear the song until after the point that he would have been well aware what the song was about.