Following the sentencing, the victim's son-in-law told the Daily Mail he was "shocked and stunned that somebody could intend on doing such a thing".
The victim, who was so frail she could barely communicate, had been sleeping on a hospital trolley in the front room of his home, having been left virtually bedridden since suffering a fall, he added.
A court heard drunken Willis had stumbled into the property at 12.30am by trying an open door as a family gathering wound down.
The victim, a former medical secretary and wartime Wren who cannot be named for legal reasons, was asleep when father-of-five Willis crept in as other relatives chatted in another room last August, prosecutor Jonathan Straw said.
When the woman's granddaughter noticed the front door was open relatives went to investigate, and found Willis - still wearing his boxer shorts but having exposed himself - on the woman's bed.
The prosecutor said: "The victim was in bed, The covers had been pulled back and her pyjama bottoms had been pulled off.
"Had he not been disturbed, as mercifully he was, he was going to rape that 94 year old lady in that bed."
Willis, who turns 26 today, was chased out of the house in Grantham, Lincolnshire, but returned soon afterwards while the woman's relatives were still on the phone to police.
Officers arrived moments later and arrested him nearby, discovering a set of car keys from the property in his pocket.
Willis, of Grantham, has convictions for theft, burglary, criminal damage, taking a motor vehicle without consent and possession of an offensive weapon dating back to 2003 when he was a juvenile.
At the time of the attempted rape he was on bail for another burglary committed in December 2015, which he later admitted.
In relation to the August 2016 incident, he denied attempted rape as a result of the incident but was found guilty by a jury following a trial. He had earlier admitted burglary in relation to the car keys.
Straw told Lincoln Crown Court the great-grandmother-of-four had moved back to the house to spend her final months in the company of relatives, after a stint in respite care at a local care home.
During Willis's trial, jurors were told the victim has passed away four months after the incident - although her death was not connected with the attempted rape.
Mark Watson, mitigating, said Willis apologised to the victim and her family for what he did.
The barrister added: "He was plainly under the influence of something. There is nothing about the offence that he can remember.
"It may be due to alcohol but it may be that he is unable to bring himself to believe what happened."
Following the case, the victim's son-in-law said the family now wanted to try to put the "horrific episode" behind them.
He added: "She was a very frail person asleep in bed who had already entered the end of life phase - she didn't talk very much and although we could take her out in a wheelchair it was becoming increasingly difficult.
"We don't know how much of what happened registered with her - we deliberately didn't raise it with her afterwards and we just have to hope she didn't suffer in any way.
"We didn't think it appropriate to put her through the ordeal of a medical examination.
"I was the only adult male in the house at the time and when I confronted him the adrenaline took over and I didn't initially appreciate what was happening.
"Thankfully he ran away without becoming violent and it was only later that the gravity of what he was about to do hit me.
"No, I just want to eradicate that night from my mind - at least while he is in prison."
He said his mother-in-law, who had two children and two grandchildren, had been widowed for over 20 years. Her late husband was in the Royal Navy before working as an insurance assessor.
