Bells and whistles
Attractive additional features or fittings
'Bells and whistles' appeared many times in 18th and 19th century texts in literal references to warnings or promotional events. These contexts included citations about fire engines, the Salvation Army, circuses; anyone in fact that was trying to draw attention to themselves might do so using a bell or a whistle. The current meaning of 'bells and whistles' is different. It refers to items that have a full array of additional features and extras. Cars and computers are just the right type of products to sport these additional features and it comes as no surprise to learn that it is in those technological fields that the phrase originated. The earliest printed reference that I've found to the current meaning of the phrase 'bells and whistles' is in a classified advert for a 1969 Buick Riviera, in the Wisconsin newspaper The Capital Times, June 1971:
69 Riviera: One owner and driven very few miles, with all the bells and whistles, $3695.
It wasn't long before computer salesmen got in on the act and started to offer computers with 'all the bells and whistles'. An example from that context from later in the 1970s comes from the US computer magazine Computerworld, August 1976:
Before even considering bells and whistles, a user should look at the plain vanilla system and see just how operationally sound it is.
So why was 'bells and whistles' chosen as synonymous with 'the full complement of accessories'? After all, the expression bears little relation to the earlier 'making a lot of noise' meaning. It is possible that, by naming two items with a similar function, the expression just suggested excess, something akin to 'belt and braces'.
The derivation of 'bells and whistles' isn't definitively known so, while it isn't my preferred method to speculate, there isn't much option in this case but to offer plausible derivations and continue to look for better evidence.
The coiners of the present meaning of the term probably weren't thinking about fire engines etc. when they adopted the new meaning. One possible source for the phrase is fairground organs. These were somewhat like what a one-man band would resemble if he formed an army. It hardly does them justice to call them musical instruments; they were orchestral automatons on an industrial scale and bristled with every form of instrument that could be banged, shaken or blown. The excess implied in 'bells and whistles' fits well with the image of fairground organs.
Another possibility is that the expression derived from the work of the English cartoonist and sculptor Rowland Emett. He created cartoons and contraptions, in a similar eccentric and whimsical style to the earlier artist, William Heath Robinson. Given that 'bells and whistles' appears to be an American phrase, the archetypally English Emett might seem a dubious source. What places him at the front of the queue is that he became celebrated in the USA after he designed Chitty Chitty Bang Bang in 1968. This places him as the designer of a very well-known and unambiguously 'bells and whistles' creation at the right place at the right time. It does seem quite likely that the unknown American who coined the phrase 'bells and whistles' in or around 1971 would have had Emett's designs in mind. Plausibility isn't proof though; we'll just have to wait for that.